To figure out your average CPM across different ad networks, you need to compare the total amount you spent with the total number of impressions you received. CPM means cost per thousand impressions, so divide your total spend by your total impressions, then multiply the result by 1000. If you’re using more than one ad network, do this for each one and then add up all the spend and all the impressions. Once you get the totals, do the same calculation again to find the overall average. This gives you a better picture of which networks give the best value for your money. Has anyone tried comparing CPM across networks to guide spending?