SitePoint (I believe) charges $35 per thousand impressions...however I think SitePoint likely offers a much more loyal user base (IE: more interest in the ads) than other sites. If the site has the same subject as yours, then you can definetly charge more than if it were some affiliate banner.
Thanks for the replies guys. I haven't really approached any companies about advertising on my site. They've all come to me, thus far. There is certaintly plenty of interest, I just need to make sure I'm charging right.