How it works
It answers the question “what does a buyer cost”. Unlike ROAS it does not depend on the size of the order, so in shops with a wide price spread it is read together with the average order value, not instead of it.
Formula
Spend ÷ number of conversions
Example
The same shop: spend 40,000, 300 orders.
Cost per conversion = 40,000 ÷ 300 = 133. At an average order value of 600 that is about 22% of the order.