I have two button spots, and a number of other advertising opportunities, available at the end of the month for my site http://www.helpwithhosting.com
The ad buttons go for $35/mo. and you should receive around 10,000 impressions. I might also be willing to sell by the impression if the ad spots don't get sold before the 1st. This pricing would be $6/CPM.
Right, the button ads would be the top row. The sidebar ads and the full banner ads at the bottom of the page are available on a per impression basis. You can take a look at http://www.helpwithhosting.com/advertising.htm for more specifications on the ad spots, rates, and restrictions.