Opportunity Cost and Thinking at the Margin
Two ideas behind almost every economic decision: opportunity cost, the value of the best alternative given up, and marginal thinking, comparing the extra benefit of one more unit with its extra cost. Includes economic profit, why sunk costs don't count, diminishing returns, and what these ideas mean for companies and investors.
The short version
Every choice has an opportunity cost: the value of the best alternative given up. For a company, that means profit only creates value if it beats what investors could earn elsewhere at similar risk; a business earning 7% on capital that costs 9% is shrinking its owners' wealth while reporting a profit. Marginal thinking compares the extra benefit of one more unit with its extra cost, which is why an airline can rightly sell an empty seat below its average cost. Money already spent can't be recovered, so it shouldn't drive the next decision. And extra inputs usually add less and less, which puts limits on how far any business can grow by doing more of the same.
By the end of this lesson, you'll be able to
- Define opportunity cost and use it to distinguish economic profit from accounting profit
- Apply marginal reasoning to pricing and output decisions, and explain why sunk costs should be ignored
- Explain diminishing returns and how these ideas apply to an investor's own decisions
Why this matters
Most economic decisions are choices among limited options. Two ideas do most of the work in making those choices well, and both are easy to state and easy to forget. Opportunity cost asks what you give up. Marginal thinking asks what one more step adds. Together they explain why reported profits can mislead, why companies price the way they do, and why an investor's best question about any holding is what the money could be doing instead.
Opportunity cost
The Opportunity cost. What you give up by choosing one use of money over the next-best alternative. Glossary of a choice is the value of the best alternative you give up to make it. It includes costs that never appear as a payment.
- An owner who works in their own shop gives up the salary they could earn elsewhere, even if they pay themselves nothing.
- A company that uses a building it owns gives up the rent it could collect by leasing it out.
- An investor who holds cash gives up what the money could earn. In mid-October 2026, three-month Treasury bills yielded about 4.1%,1 so money held in a checking account paying nothing was giving up roughly that.
Economic profit
Accounting profit subtracts the costs a company pays. Economic profit. Profit after subtracting the cost of the owners' capital: what they could have earned elsewhere at similar risk. A business can show an accounting profit and still have negative economic profit. Glossary also subtracts the opportunity cost of the capital its owners have tied up, which is the return they could earn elsewhere at similar risk, the Cost of Capital.
| Company A | Company B | |
|---|---|---|
| Capital invested by owners | 100 | 100 |
| Net profit | 14 | 7 |
| Return on capital | 14% | 7% |
| Owners' required return at this risk | 9% | 9% |
| Cost of the capital: 9% × 100 | 9 | 9 |
| Economic profit | +5 | −2 |
Both report a profit. Only A earns more than its owners could get elsewhere. B's owners would be better off, on these numbers, with their capital invested in something else of similar risk. Growing B by investing more at 7% would make the shortfall bigger, not smaller.
This is why the return on capital matters more than the size of profits, a point Is It Profitable? made from the accounts. A business creates value for its owners only when it earns more than its capital costs.
The rest of this lesson is for members
Still to come in this lesson: Thinking at the margin, Sunk costs, Diminishing returns, For investors, the key takeaways, and 2 quiz questions with explanations.
Membership opens every lesson in the curriculum, the capstone projects, the interactive tools, and the members-only channels in the Hyperion Entities Discord. The first lesson of every module stays free.
Go deeper
- Costs, Scale, and ProfitEconomics for Investors (depth track)
- Cost of CapitalPart 4: Research and Valuation
- Loss Aversion, Anchoring, and Sunk CostsPart 6: Strategy, Behavior, Accounts, and Taxes
- Is It Profitable?Part 3: Understanding Businesses
Notes and sources
- Federal Reserve Bank of St. Louis (FRED), from the Board of Governors of the Federal Reserve System. 3-Month Treasury Bill Secondary Market Rate, Discount Basis (DTB3), daily. Checked 2026-10-14. Back to text