Activity (real users visiting every day, post counts don't matter too much if it's not quality content -- you can use the rss feeder to just generate a ton of nonsense or bots or buy old databases), revenue if there is any.
Usually price is close to revenue for last 12 months.
If you have something else that could be potentially profitable to client, price could go up based on that. For example: good domain name, high page rank (though its not important these days), some unique software client could reuse elsewhere, etc.
If you are considering buying one, check how much work forum requires. If you need to be very active to maintain current level of revenue, you need to ether want to do that yourself or account for hiring someone to do that. Also consider possibility that after forum sale forum owner might simply open another forum with different name and all users will move there. That happens very often.
You can find out more by browsing existing and old forum sales on websites like flippa.com (that website has outrageous fees, but its good for figuring out prices)
Or a multiple of that (4x annual revenue, for example), depending upon the market, competition, and of course the value of the site to the potential buyer. An accurate and complete income-expense picture should be provided to potential buyers.
If the site doesn't make any money, just forget selling it. Give it away, or ask some small amount to cover the cost of software, etc., or just shut it down.