Marginal CPA

In one line

Marginal CPA is the cost of the conversion your next dollar of spend would generate — not the same as your average CPA so far.

Why average CPA alone is risky

"This campaign's CPA is $8, let's scale it up" is a risky call. $8 is the average across everything you've spent — not the efficiency of the next dollar. Once a spend-conversion curve starts flattening, average CPA can still look fine while marginal CPA has already gone bad.

How to judge it

Marginal CPA ÷ average CPA above 1 means the next dollar is worse than average — a saturation signal. Close to 1 means there's still room to scale.

Go deeper

Using marginal CPA to diagnose saturation is covered in Campaign Saturation Signals.