Unlike other Adnetworks, the metric used by Google for Publishers is RPM, a concept that sometimes can generate confusion. This metric system (or Pricing Model or Rate) is not used by advertisers to buy ads (Purchase pricing model: CPM – CPC – CPA) as well as by Adnetworks (Payment pricing model to publishers: CPM).
Most of Publishers do not realize that RPM Pricing Model (“Rate”) is a page-based model, while CPM is based on single ad banner and/or textual ad.
WHAT IS RPM RATE?
RPM stands for “Revenue per Mille” that is “Revenue for 1000 impressions” of a webpage.
It is much easier to think of it as the revenue of a page.
If you have, for instance, 4 ad spaces on your web page, your RPM will be higher than if you had only 2. RPM is not how much you have actually earned; it is calculated by dividing your estimated earnings by the number of page views (impressions) or opening page request: this result should be multiplied by 1000.
RPM Rate = (Estimated earnings / Number of page views) * 1000
(source Support Google)
WHAT IS CPM RATE?
CPM stands for “Cost per Mille” and is based on the price of each banner space (Ad Unit Rate).
It is the price that the advertiser pays to have a banner shown for 1.000 times.
It is calculated as the current cost (not estimated) of the advertising and represents the pricing model commonly used for online ad banners (Ad Units).