It can be very low, depending on which impressions you're buying. Not all impressions are equal, and more importantly, some impressions are worth more to some a

It can be very low, depending on which impressions you're buying. Not all impressions are equal, and more importantly, some impressions are worth more to some advertisers than others. The more finely-targeted your ads are, the higher your CPM will be. Note that "targeting" can mean serving ads to a narrow audience segment on a large network, or it can mean running indiscriminate ads on a small site with a very specific audience, or anywhere in between. "Windows Phone 7 users" is a very specific audience, but I don't think it's an incredibly valuable one compared to the broader segment of "mobile gamers," so they won't get great $CPM unless advertisers can segment their users.