Industry Cost Conditions: Constant, Increasing and Decreasing

Entry can change input prices and external economies.

Compare Two Long-Run Cases

Use the competitive long-run benchmark: identical price-taking firms, free entry and exit, a lasting demand change and no separate technology shock. Firms cover all economic costs, including opportunity costs, at the endpoint. Industry expansion may change input costs available to every firm; the type of industry tells you how. These results describe the stated model, not every real market.

Connect Price with Minimum ATC

With unchanged firm costs, entry can restore the initial zero-profit price. If industry expansion raises firms’ minimum ATC, the new long-run price must cover that higher cost. If expansion instead lowers minimum ATC, a lower zero-profit price can support the larger industry. Zero economic profit alone does not tell you whether the new price equals the old one.

Identify the Industry-Wide Cost Effect

Ask what happens to an otherwise comparable firm as the industry grows. Competition for a scarce specialized input can raise its input price. A larger supplier network can lower the delivered cost of an input available to many firms. Those are industry-wide effects on the firm’s cost schedule; neither requires that the individual firm become larger.

Compare Three Industry Types

Separate Market and Firm Output

In the unchanged-cost identical-firm benchmark, entry changes seller count while each firm returns to its original minimum-ATC output. Market output and one firm’s output therefore need separate labels: Q for the industry and q for a firm. With a cost shift, calculate the new efficient q instead of assuming it must change or must stay fixed.

Check Whether Efficient Scale Changes

An industry-wide cost increase may change both the minimum ATC and the output at which it occurs. The direction of the efficient-scale change depends on the cost function. The numerical example below deliberately shifts costs without changing the minimizing q, showing why a higher price need not mean a larger firm.

Label the Curve You Are Reading

Long-run industry supply connects equilibria after entry, exit and relevant industry input-market adjustments. One competitive firm still takes the prevailing output price as given. Its horizontal demand line and its own MC curve answer different questions from the industry’s long-run supply schedule.

Test the Scarce-Input Mechanism

As a hypothetical case, suppose expansion increases demand for engineers whose services remain scarce over the relevant range. Higher wages can raise firms’ costs and the zero-profit price. This illustrates a mechanism; it is not a measured claim about all renewable-energy markets or a prediction that every input price must rise.

Distinguish External and Internal Economies

External economies differ from economies of scale within one firm. A firm’s own larger plant can lower its cost through internal scale economies. An external economy lowers a firm’s cost because the whole industry expands, even if that firm remains the same size. Identify the level causing the saving.

Use the Stated Assumptions

Read the stated cost assumptions before deciding the endpoint. If they are missing, state the conditional result rather than claiming that every economics exam defaults to a constant-cost industry. Entry by itself does not prove that inputs became more expensive; the reason for the cost change matters.

Keep Firm MC Separate

Constant industry costs do not mean flat MC for a firm. In the example below MC rises with q even in the constant-cost case. Conversely, an external cost saving as Q grows does not mean a firm can lower its own cost simply by expanding alone. Keep the two output variables separate.

Check the Relevant Range and Equilibrium

A downward-sloping long-run supply schedule also needs a compatible demand curve and an equilibrium-selection assumption. Do not extrapolate a local cost relationship indefinitely or assume that any intersection is stable. The worked example states its output range and checks a demand curve that is steeper than the decreasing-cost schedule.

Trace the Endpoint in Order

Start from the original long-run equilibrium, identify the lasting demand shift, then determine how expansion affects minimum ATC. Use the changed costs to find the new zero-profit price and the demand curve to find Q. Finally calculate firm output and seller count. A higher final price can coexist with zero economic profit because the economic costs changed.

Read the Underlying Entry and Exit Model

OpenStax’s entry and exit lesson discusses how industry growth can change firms’ costs. The following equations, numbers and practice questions are original teaching examples, not adapted textbook exercises.

Calculate Three Different Endpoints

Let one firm’s total economic cost be TC = q² + 100 + c(Q)q. Each small price-taking firm treats industry output Q and the industry input-cost component c(Q) as given when choosing its own q. These dollar costs include the normal return required to remain in the industry. There is free entry and exit among identical firms.

For positive q, ATC = q + 100/q + c(Q). Its minimum is 20 + c(Q) at q = 10, since ATC − [20 + c(Q)] = (q − 10)²/q. MC = 2q + c(Q), so the competitive condition P = MC also holds at that minimum. A changed input-cost component shifts the zero-profit price while efficient output remains ten in this particular model.

Initially P = 60 − Q/15. All three cases share c(300) = 20, so Q = 300, P = $40, q = 10 and thirty firms form the initial equilibrium. Now shift demand to P = 70 − Q/15. Compare three hypothetical cost relationships, used only over Q = 300 to 600: constant c(Q) = 20; increasing c(Q) = 20 + (Q − 300)/30; and decreasing c(Q) = 20 − (Q − 300)/30. The input component stays positive throughout this range.

Set the new demand price equal to minimum ATC. Constant costs give 70 − Q/15 = 40, hence Q = 450. Increasing costs give 70 − Q/15 = 40 + (Q − 300)/30, hence Q = 400 and P = $43⅓. Decreasing costs give 70 − Q/15 = 40 − (Q − 300)/30, hence Q = 600 and P = $30. Divide Q by ten to obtain the seller counts. All three outcomes have more output than the initial 300; their prices move differently.

Check Profit and the Decreasing-Cost Intersection

Why does a price of $43⅓ not imply excess profit here, and why is the decreasing-cost intersection usable over the stated range?

Try Three Industry Cost Questions

Connect Costs with Entry and Market Structure

Follow the short-run and long-run adjustment timeline, compare barriers that limit entry, or review the CLEP Microeconomics learning hub.