Dude, where's my AdSense revenue?

I was never going to become rich from my AdSense income, but I used to get a small amount of spending money from it. But over the last few months, even though I'm getting more hits to my sites, I'm getting less and less money from ads. I think it's down to the fact that I'm getting less money per click. There was a time when some clicks would earn me a dollar or more (which, to be fair, seems ridiculous) but now I'm lucky to get more than 20 cents per click. So I guess the question is whether there is a general downward trend in prices being paid for clicks or if Google is taking a bigger slice of the action. Of course it could be just me who's suffering from this and it may be something specific to my sites causing the lack of cash.