Using CPM to compare ad placements
Because CPM normalises cost to a fixed number of impressions, it lets you compare placements with very different reach on equal footing. A lower CPM means cheaper reach, but always weigh it against audience quality and engagement. A higher CPM on a well-targeted audience can deliver better results than cheap, untargeted impressions.
CPM, CPC and CPA
CPM charges for impressions, cost per click (CPC) charges for clicks, and cost per acquisition (CPA) charges for conversions. CPM works well for awareness campaigns where the goal is reach; performance campaigns focused on actions are often measured with CPC or CPA instead.