Tray,
It sounds like a good opportunity however for only having 200 people logging in every day I think 500 may be a little high. For obvious reasons your not going to say the url of the site here, but what type of forum is it? In order to generate over $500 in revenue you will need to get alot of sign-ups in that 30 days.
I would also look at any other advertisers on the site (if any) and contact them to get a feel for their
conversion ratios. If you are the first one and only advertiser, then you are essentially the guinea pig. I would tell the guy that you will pay half of that $250 for the first month to get a feel for how much traffic he generates and what your
conversions are. If after a month you see good
conversions and sign ups then tell him you will continue to advertise and pay the full price of $500.
If he is a savvy business person he is obviously trying to see if he can get top dollar from you. Also if he doesn't have any other advertisers right now then he would be silly to pass on that opportunity.
Feel free to PM me if you want to chat further about this in a more private setting.