Ben Brooks:

...if you get a popular post two things will happen:

1. You will get those annoying emails Matt references.
2. Someone will outright steal your content and post it on their site.

You should click through to the article by Matt Gemmell that Ben links to. I'll put it here too in case you're feeling lazy.

I've never been asked, because nobody reads this site, but for the curious I'd charge $25,000 for a banner ad and $5,000 for a sponsored post. Prices are pro-rated to reflect said lack of popularity.