Yeah, what Linda said.
Some topics will have higher CPM because advertisers will pay big money to have their ads seen. For instance, car insurance advertisers can pay a whole lot more than free knitting pattern advertisers.
A good way to know what to expect is to check the CPM for your kewords in the Google AdWords Keyword Tool. That measures approximately how much an advertiser will pay when someone clicks on their ad for that keyword.
Car insurance CPMs are often $20 or more. Free knitting patterns? I haven't checked, but I doubt they'd be over $1 per click. There's just not a lot of money to be made there, so advertisers can't pay as much for those keywords.
So CPM changes all the time, and so does our pay rate. It's a constantly moving target.