Cost per mile could be great metrics to compare costs of your campaigns priced by 1,000 impressions. I’ve made quick calculator which should helps you. If you want to calculate CPM by your own you could just divide costs by impressions.
First CampaignTotal Cost:
Second Campaign for comparisonTotal Cost:
The calculation of CPM is simple:
CPM = (costs / impressions) * 1000
Where should I use CPM?
CPM could be useful metric to compare how much costs you display campaigns or how much effective they are. Lower CPM mean that you get more impressions with lower price. But also be aware unnecessary focus on reducing CPM could result in you reaching low quality users/traffic and this will lead to your display campaigns being ineffective.