Welcome to WebmasterWorld Guest from 22.214.171.124
Starting with the Adsense price you could offer a quantity of page impressions at the Adsence price, and then create a per-click charge on top of that. I don't have any examples of someone doing this already, but it seems like you have an opportunity to sell page impressions and then make a bit more on top of it with the clicks.
Best of luck,
Google takes a cut of the total sale to do all those things for you and gives you the rest.
If you are going to sell ad space directly you need to charge more than you get from AdSense so you get compensated for the hassles of direct sales.
Twice what you get from AdSense for the same size ad in the same spot on the page might be a good place to start.