From my experience and seeing advertising on large forums like that (meaning over 10k users and a few hundred people online at any given time) I have seen a bit of a range in prices.
But here are some I've seen:
480x60 Banner, top of page: $25 CPM (cost per 1,000 impressions) or $500-1200/month (unlimited impressions)
480x60 Banner, bottom of page: $15 CPM or $250-700/month
120x60 Buttons, anywhere: $25-100/month
Again, these are just some prices I've seen at some large forums. If this will be the only ad on your site, they would be exclusive so you may be able to get a bit more, or less, just depends on the company asking.
