Module · Thinking at the margin

What does one more cost?

Lesson 1 of 5 · 6 min

You make a product. It costs you $24 in materials and labour for each one, and you carry $6,000 a month of rent, salaries and software whether you sell one or a thousand. Last month you sold 500 at $40.

A buyer calls. They want 100 units, one-off, and they will pay $30 each.

Finance says no. Here is their arithmetic, and it is correct:

\text{average cost} = \frac{\$6{,}000 + 500 \times \$24}{500} = \frac{\$18{,}000}{500} = \$36

Every unit you made last month cost you $36, all in.

$30 is less than $36. Selling below cost. Refuse the order.

This reasoning costs the company $600, and it is worth understanding exactly why.

The fixed cost is already gone

The $6,000 does not care about the order. You pay it if you accept, you pay it if you refuse. It is not part of the decision — it has already been decided.

So the real question is not what did a unit cost? It is: what changes if I say yes?

The only rows that move are the ones the decision touches. Fixed cost sits there unchanged — which is precisely why it does not belong in the answer.

	Refuse the order	Accept the order	Difference
Units made	500	600	+100
Variable cost	$12,000	$14,400	+$2,400
Fixed cost	$6,000	$6,000	no change
Revenue	$20,000	$23,000	+$3,000
Profit	$2,000	$2,600	+$600

Units made

Refuse the order: 500
Accept the order: 600
Difference: +100

Variable cost

Refuse the order: $12,000
Accept the order: $14,400
Difference: +$2,400

Fixed cost

Refuse the order: $6,000
Accept the order: $6,000
Difference: no change

Revenue

Refuse the order: $20,000
Accept the order: $23,000
Difference: +$3,000

Profit

Refuse the order: $2,000
Accept the order: $2,600
Difference: +$600

One more unit costs $24 to make and brings in $30. That is $6 of profit, a hundred times over.

Two costs, two jobs

Both are true at the same time. Only one answers a question about the future.

	What it is	What it is for
Average cost	total cost ÷ units — $36	reporting. It tells you how last month went.
Marginal cost	the cost of one more unit — $24	deciding. It tells you what to do next.

Average cost

What it is: total cost ÷ units — $36
What it is for: reporting. It tells you how last month went.

Marginal cost

What it is: the cost of one more unit — $24
What it is for: deciding. It tells you what to do next.

The rule: do it while the extra benefit is bigger than the extra cost. Not the average benefit — the extra one.

Where this bites

The same shape turns up every time someone divides a total by a count and then makes a decision with the result:

The empty seat. A flight costs $30,000 to operate and carries 150 people — $200 a seat. Ten minutes before the doors close, an empty seat costs the airline almost nothing to fill. Any price above the cost of one more meal beats flying it empty.
The extra student. A course costs $50,000 to build and 200 people take it — $250 each. The two-hundred-and-first pays for itself the moment they cover the payment processing.
The rush job. "We can't take it, we'd be working below our hourly rate." If the team is paid anyway and would otherwise be idle, anything above the extra materials is money you did not have.

The mirror-image mistake. Marginal thinking says yes a lot, so it is easy to over-apply. It holds only while the fixed cost really is fixed. Fill the factory and the next order needs a second shift — and that shift is a new cost the decision does cause. Should we run the discount? ends on exactly that question.

The habit

Whenever someone justifies a decision with a number that came from dividing, ask what changes if we say yes. Then count only the things that move.

Next: the cost that never appears on any invoice — and the reason the second hour of anything is worth less than the first.