Make-or-Buy Decisions — Make make-or-buy decisions using relevant cost analysis - Cost Accounting

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Historical Context & Motivation

The question of whether a firm should produce a component internally or purchase it from an outside supplier is as old as manufacturing itself. During the early stages of the Industrial Revolution, vertically integrated factories handled virtually every step from raw material to finished product, in large part because reliable supplier networks did not yet exist. As markets matured and transportation infrastructure improved throughout the nineteenth and twentieth centuries, specialization became economically viable, giving rise to a formal analytical framework for what cost accountants now call the make-or-buy decision. Understanding this decision's evolution reveals why relevant cost analysis — rather than full absorption costing — became the preferred tool for short-run outsourcing choices.

1910s • Ford's Vertical Integration

Henry Ford's River Rouge complex epitomized full vertical integration, producing everything from steel to glass in-house. The implicit 'make' decision was driven by supply uncertainty and a desire for total quality control.

1950s • Rise of Management Accounting

Cost accountants began distinguishing between relevant and irrelevant costs for short-run decisions, formalizing the differential analysis framework that underpins modern make-or-buy evaluations.

1980s • Outsourcing Wave

Japanese lean manufacturing and the growth of global supply chains made outsourcing strategically attractive. Companies like General Electric adopted rigorous make-or-buy models incorporating both quantitative costs and qualitative factors.

2000s–Present • Strategic Sourcing & Total Cost of Ownership

Modern firms blend relevant cost analysis with total-cost-of-ownership models that account for logistics, quality risk, intellectual property protection, and supply-chain resilience in an era of global disruption.

The central question that make-or-buy analysis addresses is deceptively straightforward: Should the firm manufacture a component itself or purchase it externally? Answering it correctly, however, requires isolating only those costs that will actually change as a result of the decision — the relevant costs — while excluding sunk costs, allocated overhead that will not disappear, and other figures that cloud the analysis. This section traces the intellectual journey from gut-instinct sourcing to the rigorous, data-driven framework you will master throughout this lesson.

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Core Principles & Definitions

Before diving into calculations, it is essential to establish the conceptual vocabulary that governs make-or-buy analysis. Every make-or-buy decision hinges on the distinction between costs that are affected by the decision and costs that remain unchanged regardless of the chosen path. A relevant cost is a future cost that differs between alternatives; an irrelevant cost is one that remains the same no matter which option management selects. Misclassifying costs — particularly allocated fixed overhead — is the most common source of error in these analyses.

Relevant Costs

Future costs that differ between the make and buy alternatives. They include avoidable variable costs, avoidable fixed costs, and the opportunity cost of using capacity for another purpose.

Irrelevant (Sunk) Costs

Past expenditures or future costs that do not change between alternatives. Depreciation on existing equipment and allocated corporate overhead that persists regardless of the decision are classic examples.

Avoidable vs. Unavoidable Fixed Costs

An avoidable fixed cost is eliminated if the firm stops making the component (e.g., a supervisor's salary). Unavoidable fixed costs persist regardless and should be excluded from the analysis.

Opportunity Cost

The benefit foregone from the best alternative use of the capacity or resources freed by choosing to buy. If idle capacity can generate contribution margin elsewhere, that potential income is a relevant cost of the make alternative.

Qualitative Factors

Beyond quantitative analysis, managers consider quality control, supplier reliability, intellectual property risks, lead-time flexibility, and workforce morale — factors that may override a purely numerical recommendation.

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KEY TAKEAWAY

Think of a make-or-buy decision like choosing between cooking dinner at home and ordering takeout. You would not factor in the cost of your kitchen, because you already own it and it does not disappear if you order takeout (that is an unavoidable fixed cost). You would compare the incremental grocery cost plus your time (opportunity cost) against the price of the takeout order. Only costs that actually change with your choice are relevant to the decision.

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Visual Explanation — The Make-or-Buy Decision Framework

The flowchart above illustrates the structured process for a make-or-buy decision. Begin by identifying all costs associated with both alternatives, then systematically separate relevant costs from irrelevant costs. The relevant cost of making includes direct materials, direct labor, variable overhead, avoidable fixed overhead, and opportunity cost. The relevant cost of buying includes the purchase price plus any incremental costs such as shipping or incoming inspection. The final comparison drives the quantitative recommendation, which should then be tempered by qualitative considerations.

The flowchart above captures the essence of make-or-buy analysis: it is fundamentally a differential (incremental) cost comparison between two mutually exclusive alternatives. A frequent pitfall is including the full per-unit cost reported on internal manufacturing cost sheets, which typically spreads total fixed overhead across all products using a predetermined rate. When a manager evaluates whether to outsource a single component, much of that allocated overhead will continue regardless — the building's lease does not evaporate, nor does the plant manager's salary disappear. Including such costs inflates the apparent cost of making and biases the decision toward buying. The discipline of relevant cost analysis corrects this bias by stripping the analysis down to only those costs that genuinely differ between the alternatives.

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Mathematical Framework

The quantitative core of the make-or-buy decision can be expressed through a series of straightforward equations. The goal is to compute the total relevant cost under each alternative and choose the lower-cost option. We begin by defining the relevant cost to make a component, then the relevant cost to buy, and finally the net advantage (or disadvantage) of making.

RELEVANT COST TO MAKE (PER UNIT)

RC_Make = DM + DL + VOH + (Avoidable FOH ÷ Q) + OC

DM = direct materials per unit; DL = direct labor per unit; VOH = variable overhead per unit; Avoidable FOH = total fixed overhead eliminated if the firm stops making the component; Q = number of units; OC = opportunity cost per unit (contribution margin from alternative use of freed capacity).

RELEVANT COST TO BUY (PER UNIT)

RC_Buy = P + IC

P = purchase price per unit from the external supplier; IC = any incremental costs per unit incurred when buying (e.g., inbound freight, receiving inspection, rework for fit).

NET ADVANTAGE OF MAKING

Net Advantage = RC_Buy − RC_Make

A positive result means the buy alternative is more expensive, so making is favored. A negative result means making costs more, so buying is favored. If the result is zero, qualitative factors should break the tie.

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Common Mistake

Students often include allocated common fixed overhead in the cost to make. If this overhead will continue regardless of the decision — for example, the depreciation on a building that houses multiple product lines — it is irrelevant and must be excluded. Including it artificially inflates the make alternative and leads to an incorrect recommendation.

When opportunity cost is present, the analysis shifts meaningfully. Suppose the freed capacity could be used to manufacture a different product that generates a contribution margin of $5 per unit. Each unit the firm continues to make in-house implicitly costs an additional $5 in foregone profit, and this amount must be added to the relevant cost of making. Conversely, if the freed capacity would simply sit idle, the opportunity cost is zero. Identifying whether alternative uses for capacity exist is therefore a critical early step in any make-or-buy analysis.

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Cost Classification for Make-or-Buy Analysis

The most intellectually demanding step in a make-or-buy analysis is correctly classifying each cost element as relevant or irrelevant. The table below provides a systematic guide, and the diagram that follows illustrates how different cost layers stack up under the two alternatives, making it visually clear which layers disappear when the firm elects to buy externally.

Cost Element	Relevant to MAKE?	Relevant to BUY?	Rationale
Direct Materials	Yes — incurred only if making	No — not incurred	Avoidable variable cost of in-house production
Direct Labor	Yes — incurred only if making	No — not incurred	Avoidable variable cost; may differ if labor is contract vs. salaried
Variable Manufacturing OH	Yes — incurred only if making	No — not incurred	Varies with production volume
Avoidable Fixed OH (e.g., supervisor salary)	Yes — eliminated if firm stops making	No — eliminated	The specific fixed cost that would be saved by outsourcing
Unavoidable Fixed OH (e.g., factory lease)	No — continues regardless	No — continues regardless	Same under both alternatives → irrelevant
Purchase Price	No — not incurred	Yes — cost of the external part	Only arises under the buy alternative
Opportunity Cost	Yes — added to make cost	No — capacity is freed	Contribution margin from alternative use of released capacity

Classification of cost elements in make-or-buy analysis

The stacked-bar comparison shows the relevant cost layers for each alternative. Solid blocks represent costs included in the analysis. The dashed block at the bottom of each column represents unavoidable fixed overhead, which appears on both sides and is therefore irrelevant. The decision favors whichever alternative has a lower total of solid (relevant) cost blocks.

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Worked Example

Apex Electronics manufactures 20,000 circuit boards per year for use in its flagship product line. An external supplier, GlobalParts Inc., has offered to supply the same board for $18 per unit. Apex's management accountant has prepared the following per-unit cost report for internal production:

Cost Element	Per Unit
Direct Materials	$6.00
Direct Labor	$4.00
Variable Manufacturing Overhead	$2.50
Fixed Manufacturing Overhead (allocated)	$5.50
Total	$18.00

Apex Electronics — Per-Unit Manufacturing Cost Report

Additional information: Of the $5.50 fixed overhead per unit, $2.00 consists of a production supervisor's salary that would be eliminated if the boards are outsourced (avoidable). The remaining $3.50 is allocated plant-level overhead (building depreciation, insurance) that will continue regardless. If Apex stops making the boards, the freed capacity could be leased to another company for $30,000 per year.

Should Apex Make or Buy the Circuit Boards?

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Step 1 — Identify Relevant Costs to Make

The relevant costs of making include direct materials ($6.00), direct labor ($4.00), variable overhead ($2.50), and the avoidable fixed overhead ($2.00). The unavoidable fixed overhead of $3.50 per unit is excluded because it persists regardless of the decision.

Relevant manufacturing costs per unit = $6.00 + $4.00 + $2.50 + $2.00 = $14.50
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Step 2 — Compute Opportunity Cost

If the firm stops making the boards, it could lease the freed capacity for $30,000 per year. Spread over 20,000 units, the opportunity cost per unit is $30,000 ÷ 20,000 = $1.50. This is added to the relevant cost of making.

Opportunity cost per unit = $1.50
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Step 3 — Total Relevant Cost to Make

Add the avoidable manufacturing costs and the opportunity cost.

RC_Make = $14.50 + $1.50 = $16.00 per unit
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Step 4 — Total Relevant Cost to Buy

The purchase price from GlobalParts Inc. is $18.00 per unit. No additional incremental costs (shipping, inspection) are mentioned, so the relevant cost to buy is simply the purchase price.

RC_Buy = $18.00 per unit
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Step 5 — Compute Net Advantage and Decide

Net Advantage of Making = RC_Buy − RC_Make = $18.00 − $16.00 = $2.00 per unit. Over 20,000 units, the firm saves $40,000 annually by continuing to make the boards.

Decision: MAKE — saving $2.00 per unit ($40,000 per year)

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Note on the Full-Cost Trap

At first glance, the full cost to make ($18.00) equals the purchase price ($18.00), suggesting indifference. However, $3.50 of the full cost is unavoidable overhead that will persist even if Apex outsources. By stripping it out and adding the opportunity cost, we see the true relevant cost to make is only $16.00 — and the correct decision is to make. This example underscores why using relevant cost analysis rather than full absorption costing is essential.

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Qualitative Factors & Limitations

While quantitative analysis provides the numerical backbone of a make-or-buy decision, experienced managers know that numbers alone rarely tell the whole story. Qualitative factors can reinforce, modify, or even override a purely cost-driven recommendation. A component may be cheaper to outsource, yet the risk of losing proprietary knowledge or becoming dependent on a single supplier may justify the higher cost of internal production. Conversely, a component that is cheaper to make might still be outsourced to free management attention for higher-value strategic activities.

Factor	Favors MAKE	Favors BUY
Quality Control	Tighter in-house control over tolerances and consistency	Supplier may have specialized expertise or certifications
Intellectual Property	Protects proprietary designs and processes	Non-core components pose little IP risk
Supply Reliability	Reduces risk of supplier disruption or stockouts	Multiple suppliers can hedge supply risk
Lead Time & Flexibility	Faster response to design changes and volume spikes	Scalable capacity without capital investment
Workforce Impact	Maintains employment; preserves institutional knowledge	Allows redeployment to higher-value tasks
Long-Run Cost Dynamics	Stable costs if learning curves continue to lower unit cost	Supplier competition may drive prices down over time

Qualitative factors influencing make-or-buy decisions

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KEY TAKEAWAY

The quantitative analysis determines cost advantage, but the final decision lives at the intersection of cost analysis and strategic judgment. Think of it like hiring a contractor to renovate your house: even if the contractor's bid is lower than your DIY material costs (unlikely, but hypothetically), you might still do it yourself to ensure the wiring is done exactly right — especially if the wiring is critical to the house's safety. In business terms, core competencies and strategic risk often deserve a premium.

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Connection to Advanced Theory

The basic make-or-buy framework you have studied is a short-run, single-period model. In practice, many outsourcing decisions have multi-year implications that require more sophisticated tools. Advanced cost accounting and strategic management courses extend the analysis in several directions, including Total Cost of Ownership (TCO) analysis, Activity-Based Costing (ABC) for more accurate overhead assignment, and Transaction Cost Economics (TCE), which examines the governance costs of market-based versus hierarchy-based coordination.

Feature	Basic Relevant Cost Model	Advanced / Strategic Models
Time Horizon	Single period (short-run)	Multi-period with discounted cash flows
Cost Measurement	Differential / incremental costs	TCO: includes hidden costs (coordination, rework, logistics)
Overhead Treatment	Avoidable vs. unavoidable split	ABC: driver-based tracing for greater accuracy
Risk Analysis	Qualitative discussion	Scenario analysis, Monte Carlo simulation, real options
Theoretical Foundation	Managerial accounting relevance concept	TCE: asset specificity, bounded rationality, opportunism

Basic vs. advanced make-or-buy frameworks

As you progress in your studies, recognize that the relevant cost model is the essential first step in outsourcing analysis, not the final word. It develops the critical habit of filtering out noise — sunk costs, allocated overhead, and other irrelevant figures — so that decision makers focus on the costs that genuinely change. Advanced models build on this foundation by widening the scope of what counts as a relevant cost and incorporating uncertainty, multi-period dynamics, and strategic alignment. The discipline of relevant cost thinking, once mastered, transfers directly to other decision contexts: special-order pricing, product-line discontinuation, capacity-constrained resource allocation, and joint-product sell-or-process-further decisions.

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Practice Problems

Question 1: CONCEPTUAL

A plant manager argues: 'Our full manufacturing cost for Part X is $22 per unit, and we can buy it externally for $20. We should obviously outsource.' Explain why this reasoning may be flawed, and describe the correct analytical approach.

Show answer

The full manufacturing cost typically includes allocated fixed overhead that will not be eliminated by outsourcing — for example, building depreciation or plant-level administration. These unavoidable fixed costs are irrelevant to the decision because they persist regardless of whether Part X is made or bought. The correct approach is to identify only the relevant (avoidable) costs of making — direct materials, direct labor, variable overhead, and any truly avoidable fixed overhead — and compare that total to the purchase price plus any incremental buying costs. If the relevant cost to make is less than $20, the firm should continue making Part X despite the full-cost figure being $22.

Question 2: BASIC CALCULATION

Bravo Manufacturing produces 10,000 units of Component Y per year. Per-unit costs are: direct materials $8, direct labor $5, variable overhead $3, and allocated fixed overhead $6 (all of which is unavoidable). An external supplier offers the component at $17 per unit. There is no alternative use for the freed capacity. Should Bravo make or buy Component Y?

Show answer

The relevant cost to make is the sum of avoidable costs: $8 (DM) + $5 (DL) + $3 (VOH) = $16 per unit. Because all $6 of fixed overhead is unavoidable, it is excluded. The opportunity cost is zero because there is no alternative use for the capacity. The relevant cost to buy is $17. The net advantage of making is $17 − $16 = $1 per unit, or $10,000 annually. Bravo should continue to make Component Y because doing so saves $10,000 per year compared to outsourcing.

Question 3: INTERMEDIATE

Crestline Corp. manufactures 15,000 units of Part Z annually. Costs per unit: direct materials $10, direct labor $7, variable overhead $4, fixed overhead $8 (of which $3 is a supervisor's salary that would be eliminated if outsourcing occurs). A supplier offers the part at $24. If Crestline stops making Part Z, the freed capacity could be used to produce Product W, which would generate a total annual contribution margin of $45,000. Should Crestline make or buy Part Z?

Show answer

The relevant cost to make consists of direct materials ($10), direct labor ($7), variable overhead ($4), and avoidable fixed overhead ($3), totaling $24 per unit. The opportunity cost of making is the foregone contribution margin from Product W: $45,000 ÷ 15,000 = $3 per unit. Total relevant cost to make = $24 + $3 = $27 per unit. The relevant cost to buy is $24 per unit. The net advantage of buying is $27 − $24 = $3 per unit, or $45,000 annually. Crestline should buy Part Z from the external supplier and use the freed capacity to produce Product W.

Question 4: APPLIED

Delta Medical Devices produces 8,000 sensor housings per year. The internal cost sheet shows: direct materials $12, direct labor $9, variable overhead $5, and fixed overhead $10 per unit. Investigation reveals that $4 of the fixed overhead is traceable to a leased machine ($32,000/year lease) that could be cancelled upon outsourcing; the remaining $6 is allocated corporate overhead. A supplier in Southeast Asia offers the housing at $27, but Delta estimates $1.50 per unit in additional inbound freight and inspection costs. No alternative use exists for the freed floor space. However, Delta's quality manager warns that the supplier's defect rate is 2%, compared to 0.5% in-house. Each defective unit costs $50 to rework. Perform the analysis incorporating the expected rework cost differential.

Show answer

Relevant cost to make: DM $12 + DL $9 + VOH $5 + avoidable fixed OH $4 = $30 per unit. Opportunity cost is zero. Now for the buy side: purchase price $27 + freight/inspection $1.50 = $28.50 per unit before quality adjustment. Expected rework cost if buying: 2% × 8,000 = 160 defective units × $50 = $8,000, or $1.00 per unit. Current in-house rework cost: 0.5% × 8,000 = 40 defective units × $50 = $2,000, or $0.25 per unit (already embedded in variable overhead, so the incremental rework from switching is $1.00 − $0.25 = $0.75 per unit). Total relevant cost to buy: $28.50 + $0.75 = $29.25 per unit. The net advantage of buying is $30.00 − $29.25 = $0.75 per unit, or $6,000 annually. Quantitatively, buying is slightly favored, but the margin is thin enough that qualitative factors — especially the quality risk and supply-chain reliability for a medical device — may tip the decision back toward making.

Question 5: CRITICAL THINKING

Echo Industries is evaluating whether to outsource a component used across three product lines. If outsourced, $120,000 of currently allocated fixed overhead would be 'saved' on internal cost reports, but only $40,000 of that overhead is truly avoidable. A consultant argues that the $80,000 in unavoidable overhead should be reallocated to the remaining in-house products, increasing their per-unit costs and potentially making them look unprofitable as well. Analyze this 'death spiral' phenomenon and explain how relevant cost analysis prevents cascading outsourcing errors.

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The death spiral (also called the outsourcing cascade) occurs when unavoidable fixed overhead is reallocated to fewer remaining products after an outsourcing decision. Each reallocation inflates the apparent per-unit cost of the surviving products, making them also appear more expensive than external alternatives and triggering additional outsourcing rounds — even though the underlying fixed costs have not changed. Relevant cost analysis prevents this cascade by requiring managers to exclude unavoidable fixed overhead from the decision calculus entirely. The $80,000 that cannot be eliminated is irrelevant regardless of how many products share it. By focusing solely on the $40,000 of truly avoidable cost and comparing it to the incremental cost of buying, the firm avoids the illusion that outsourcing reduces total costs by the full $120,000. The discipline of relevant cost thinking thus acts as a safeguard against serially dismantling in-house production based on accounting artifacts rather than genuine economic savings. Firms caught in this spiral often discover, too late, that they have outsourced their core competencies while still bearing the same overhead burden, resulting in higher total costs and diminished strategic capability.

SUMMARY

Summary — Make-or-Buy Decisions

Summary

A make-or-buy decision requires managers to compare the relevant cost of internal production — which includes direct materials, direct labor, variable overhead, avoidable fixed overhead, and any opportunity cost — against the relevant cost of purchasing externally (purchase price plus incremental buying costs). Unavoidable fixed overhead and sunk costs are excluded because they do not change between the alternatives. The option with the lower total relevant cost is the quantitatively preferred choice.

Beyond the numbers, qualitative factors — quality control, supplier reliability, intellectual property protection, lead-time flexibility, and workforce considerations — can reinforce or overturn the cost-based recommendation. The death spiral warns against reallocating unavoidable overhead to remaining products after outsourcing, which can trigger cascading and ultimately harmful outsourcing decisions. Mastery of relevant cost analysis equips you to make informed, defensible sourcing recommendations that align with both the firm's financial objectives and its broader strategic posture.