Acme Watches spends $900 for 75,000 impressions, a CPM of 900 / 75,000 x 1,000 = $12. In late November the same ad set pays $1,200 for 60,000 impressions, a $20 CPM, because more advertisers are bidding for the same people. CPA rises with it, even though the ad did not change.

CPM explains many swings in CPA and ROAS that have nothing to do with creative: seasonality, competition, audience size and placement. It also feeds every downstream cost, so a CPM jump has to be beaten by better click or conversion rates or it shows up straight in your CPA.

You cannot set CPM, but you can influence it. Broader audiences usually cost less per impression than narrow ones, and ads people respond to tend to be delivered more efficiently. Plan for higher CPMs around big sale seasons instead of reacting to them.