There is a "old music" related website that I maintain for my husband and I have populated it with google ads which are doing pretty well. I have been approached by people through a few emails asking if they can put ads on the site. I don’t know what to charge. I know it probably depends on lots of factors but is there a ball-park of what is the going rate for ads? Most of the pages on the site have a google page rank of 5.
I think it depends on how many visitors you got and how great the companies are that have contacted you. I think you could try to ask a price just a little bit more then you want for it at least and eventually you can fall back a little when they think it’s to expensive. I couldn’t say a specific price, that depends on how good your site is and the two things I called at first. Good luck!