This is a killer question. Rates are set by the market. Price is determined only by what the buyer is willing to pay, nothing else. So, a lot will be determined by your salesmanship.
But I have a bit of a trick that might be useful to you. Place an AdSense ad into the spot where you intend to sell ads directly. Create an ad channel just for that ad. That will give you CPM and earnings numbers to start with.
From there, I would bump the numbers by about 50% to adjust for the portion of revenue which you are no longer sharing with Google AdWords.
After you have been selling ads to your target customers for awhile, you will know how to adjust pricing. It's only the getting started part which is really hard.