I have a forum that I'm not interested in selling (ever), but I'd like to know how much it is worth in terms of sell-able value.

video game genre
40k members
100+ new registrations/day
200-400 online at any given time (never less than 200, maxed at 800)
~$30/day income
8-13k unique visits/day

I know that based on 1 year's income, it'd be worth about $11,000.

But what about memberships? I get a ton of organic SE traffic, and a lot of memberships. I would think that each registered user would add to the value of the site, but in which way?