Why your cost per acquisition moved
Cost per acquisition is not a setting. It is produced by three numbers — CPM, click-through rate and conversion rate — and when it moves, usually one of them is responsible. Each has a completely different remedy, and most accounts get restructured when the answer was somewhere else entirely.
Take two periods from Ads Manager and this works out which one moved. Nothing is stored, nothing is sent anywhere, and it runs in your browser.
What it checks
CPM rose, the rest held
Auction cost. Competition and seasonality, not your advertising. No setting fixes it and the decision is commercial.
Click-through fell
Fatigue if frequency rose with it. If frequency held, the creative or the audience changed rather than wore out.
Conversion rate fell
The problem is after the click — or conversions are still happening and stopped being recorded.
Under 50 conversions a week
Delivery has not stabilised, so the comparison may be describing noise. It will tell you when this applies.