How much to charge for adspace?

[pokerlink]

Hi

I have a site where people can play texas holdem, FL and NL for free. Thinking about adding adds to the play window so they show when people sit att the table and play. How much would I charge online poker sites for such an ad? Many variables Im sure, but give me a point in the right direction. The site has about 10000 people per month that play a game. The count is based on atmost 1 gae per uniqe client. So in effect perhaps more games are being played.
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On what factors should I base the cost of the ad?