Ask YC: CPM ad rates

Does anyone know a place to find CPM ad rates? Or know them offhand? We might be going into negotiations soon with an educational organization to provide a web-based productivity tool and wanted to know what kinds of CPM rates we could expect - generally - in case we need to provide the service at a discount (we'd advertise to recoup our costs).

The users will all be educators. I was going to use $10 CPM as an average.

Please let me know if I am smoking crack.