The economics of non-commercial blogging

Has anyone come across any figures on what people can expect to make if they put ads on their weblogs via Google, Yahoo or others these days? I only have one data point - a blogger with several hundred readers a week made $25 over the period of a few months. Surely by now someone has produced some kind of chart or database (or academic article?)

I don't believe most bloggers do it for the money but it would be interesting to know whether they make any noticeable sums at all...