Over on adClustr there is a post describing the different tools and services you can use to get your RSS feed to earn a bit of money through advertising. I am not sure what the average going rate is for an advertisement in a feed, but it seems to be growing in popularity to monetize them as many of the bigger sites I read have already used one system or another to implement them.
I have to admit that I find advertisements in feeds a little less glaring than most on site advertising, but at the same time, I have never clicked an advertisement in a feed.
Some of the tools and services mentioned in the article include Text-Link-Ads Feedvertising, Feedburner Ads, Pheedo and Kanoodle.
Check out the article for some details on these services, and if you have thousands of feed readers, and feel you should monetize them, then one of the mentioned sites will help make that happen.