I am starting to do some Google ads on targeted sites. How can I figure out what the optimal CPM is for a particular site?
It sucks, but there's not quick & easy way to do it. You'll have to enter your max CPM and wait and see if it generates impressions. If it doesn't you'll have to up your bid and wait some more. Not a great system by any stretch of the imagination. Back in December I did a quick write up about AdWords CPM advertising that you may find helpful.