Bull,
It’s really hard to tell. Present this information and it will be easier to place an estimation of what someone would pay:
1. # of unique visitor per month
2. % of US and non-US visitors
3. What are the main keywords that people use to get to your site?
4. Your domain name
5. players you refer to poker rooms/casinos on that site for an average month
There is no set hard and fast rule about what to charge. You don’t want to charge too much so that the casino/poker room doesn’t want to advertise on there in the future, but you don’t want to charge too little.
Mark