When evaluating the value, ask for some traffic facts and revenue figures if any. Post and Member counts should not be the main factors here. You should rather pay attention to "how active the forum currently is" and the overall content quality which matters a lot.
Take a look at revenue and traffic (sources also) for as long a time period as possible the longer the better 18 months is a good figure to shoot for. Find out where the traffic is coming from. If revenue is climbing for a long period it's best to consider the website strong / active members. If the revenue is declining you'll have to consider why and how much of a drop off is worth the investment. There are no clear numbers it's all perspective. If the site is not monetized that's another factor to consider. Just take your time and examine each possible aspect. Just like investing take as much time as possible that you are permitted sometimes taking a long time forces a seller into a bad position or reveals flaws in the investment.
Make sure the content on the website is unique and make sure it has a strong user-base. You don't want to buy a forum with hardly any active members on it. If you've checked this, then I would say around $800-1000 (MAX).
Recent updations should be a major consideration.Revenue information could help access the value more perfectly.
Also, check to see where the members are actually from. If the majority of members are Asian, then, unless the forum is specifically aimed at asians, it is probably a forum full of paid posters, meaning that as soon as you buy the forum, activity will stop. Also look at the amount of traffic. The majority of people who visit the forum won't sign up or post, so if the post count is around the same as the traffic count, again, it's probably paid posters.
I would say $300 is the fair price. But the price is depend much more than post and member,for example a forum with high PR will worth more and a good back link will also increase the forum traffic over time.
Depends... As mentioned, they may be paid posters. Also, check if the site will be approved by Adsense and other affiliate programs.