Stuart McDonald posted a thread at WebmasterWorld explaining how you can use Google Ad Manager with Google AdSense to specify a minimum eCPM.

You must keep in mind that you need to have other ads in your Ad Manager inventory to compete with your AdSense ads, in order for this to work properly. Google Ad Manager allows you to set a minimum CPM for most ad runs. By specifying the minimum CPM with house or remnant ads in Ad Manager, your AdSense ads will compete on the CPM you set and will not show for a lower CPM.

Stuart explains you will likely see that the “immediate result will probably be a drop in your AdSense income, as long as you’re running other, better paying ads (from direct advertisers, another network or whatever), your end resultant income for the total number of impressions, should be higher.”

Forum discussion at WebmasterWorld.