I am building my own custom ad server script. Media buy networks are affiliate handicapped. Their system were designed for large brand name exposure and not CPA setups.
You will never be able to bill off your numbers with that. Trust me, that's something you want to be able to do.
The good Ad Servers: Zedo, DART, Mediaplex, Atlas all work perfectly for CPA marketing. You just need to tweak your thinking slightly.
At the end of the day eCPM is all you care about with CPM buys.
If you have an eCPM of $2.00 and you're paying $1.00 CPM, you're gold. I've seen eCPM as high as $12 on $1.00 CPM buys.