How Much Should You Charge For CPM Advertising?

I have beeen using CPC advertising for years now and have been fairly successful with it making thousands of dollars per month with programs such as Google AdSense and Chitika. I want to, however, start looking at CPM advertising as I feel that several of my blogs, such as my BlackBerry Blog RIMarkable, have reached traffic levels where charging advertisors based on the number of times their ads are seen compared to the number of times they are clicked on could be lucrative.

The big question that I have is how much to charge? I am not sure even where to start.