I charge pretty high rates, but that's only because I want to detract 99% of those who pretty much want something for nothing:
I haven't gotten a single person to actually pay that rate, and that is exactly what I want. Honestly, I don't want to get into the advertisement/sponsored post game either, but if an entity out there wants it bad enough, then I'll entertain it for the right price. I'm willing to wait until that happens without getting involved in these smaller entities who want to take advantage of bloggers who'll accept almost anything for exposure.
I had someone this week express interest in a banner ad. My rates are $500/month, and this person offered $100 for THREE months.
That's the kind of stuff that I'm trying to detract.
...but, you might be different, so don't take my philosophy as gospel. You may be willing to accept less, and that's okay. We're all different.