in a way you still pay an eCPM or eCPC with a fixed time ad. The ad will get clicked just the same, and the site will sell the just the same.

The only benefit is if your site sells really well then you make off with a good profit but you wouldn't know either way till you put the ad up.

blacklabelads from Adbrite allows you to buy ad space by time, but only text ads on sites in their network. And you can rotate multiple ads to test and see which ones work best