What is CPA, and when are two CPAs not comparable?
CPA (cost per acquisition) is ad spend divided by the number of conversions the platform counted. Both halves are softer than they look: conversions is a set of actions somebody configured, and the count for any recent period keeps rising for weeks after the click. A CPA quoted without saying which actions it counts, and how complete its window is, is not comparable to any other CPA.
There is no universal definition of a conversion. Each account decides which actions count, and those choices routinely include page views, form starts, phone-call clicks and newsletter signups alongside actual purchases or booked appointments. Two campaigns in the same account can therefore report CPAs that differ by an order of magnitude purely because one is counting enquiries and the other is counting sales. Before comparing any two CPA figures, the first question is not which is lower, it is which actions each one is counting.
The second problem is time. Platforms report a conversion against the date of the click that led to it, and they keep adding conversions to past dates for weeks as delayed ones arrive. That means the last seven days always looks worse than it will eventually be. A CPA computed on a recent window is not wrong so much as unfinished, and comparing an unfinished window against a settled one manufactures a decline that is not there.
What makes a CPA decision-grade is boring: state the conversion actions it counts, state the date window and roughly how complete it is, and judge it against a cost ceiling derived from your own margin and close rate rather than against last month's number. A ceiling is a fact about the business. Last month is a fact about last month.