As I said to Den82, I recently stumbled upon an article talking about CPC (Cost Per Click) for Google Adsense. I've always wondered why I only made $.02 per click some days, while others I could make $2 to $5 PER CLICK. It's pretty easy to figure out the CPC for your keywords; you can follow these simple instructions at http://tinyurl.com/googlecpctool . I put this on my site a few weeks ago, and already I've averaged more in the last two weeks than in the last two months. It was so easy! Obviously it also matters how much traffic is coming to your site... But if you've got a steady audience, it should be pretty simple. I really hope this helps... It helped me out a lot.
While the article gives some good insights. I would say the niche matters. A niche in which there are more advertisers will obviously get more CPC.