here is the usually vague answer to your question from Google:
Home & getting started Cost-per-thousand-impressions (CPM) Next: Custom events Advertisers running CPM ads set their desired price per 1000 ads served and pay each time their ad appears.
I think a business opportunity has been missed by those who do marketplace plugins. If someone does a google Admobs widget where all I would need to do is put in the Google Admobs ID, (Google provides this and its dependent on the type of advert you want to present to your app users ...