I would factor in the amount of posts, how old the forum is and how many plugins/themes you purchased and figure out the right price to sell it for. If it's a forum with only 100 posts, 50 threads or even lower you're probably not going to get much for it. Best to sell a forum like that for maybe $40, unless it's on paid software then I'd bump up the price a little bit more. It also depends on the niche of it too, gaming forums probably won't be worth much just because the niche is very saturated unless it's a old website. Ultimately it's best to throw a price out there and see where it goes if you're really wanting to sell your website. Just gotta wing it I suppose.