You may want to look again. One of the companies neeraj1997 mentioned was JuicyAds. I use them (as an advertiser) and they are definately CPM.
They're CPC for the filler ads but they offer direct rates in their marketplace based on a CPM calculation. So you're looking at 1c/click if no one buys your ad space, with decent CPM rates (which you can set yourself or let their system figure out) when they do. It's actually a very nice system, though it'd be nice if the filler ads paid more or their passback system was more flexible!