CPMs are performance based. Starting at 70%, they can increase or decrease depending on the performance of the traffic.
By performance we mean the time each visitor leaves the ad open and the source of the traffic (Countries, Channels and Keywords). In order to really gauge your performance, it's best to test some of your sites and see what CPM you get.
Please be aware that it could take a few days for the system to adjust and get the best CPM. Also, the system checks every day performances to update CPMs accordingly.