Lol, I wish I knew for sure. I've heard others say that the "competetive-ness" of the topic might be a factor (the more of a hot topic, the higher the CPM), but I doubt this is the only determinant. Others have said the quality of the content (in the advertizer's eyes) might play a part, too. I'm guessing this means having good keyword densities for popular searches, etc. I do use Google AdWords to do a bit of this.
I only have two hubs right now, but for some reason my starting CPM started at around $9.00 and has since climbed to $11.80. I honestly don't know why. It immediately struck me as suspicious because I've heard a lot of people mention that they usually get $2-5 CPM (but then again, I wouldn't be the one to know an accurate average because I'm really new to all this).
I've only been a member for 5 days. I haven't added any other hubs or anything... I'm trying figure out if this is just a "fluke" or something I can actually capitalize on.
I wish I could give a more definitive answer.