Many would say:
"Take the monthly income, multiply it by 12-15 and then there is your Buy It Now price."
There are in fact other things to take into consideration.
1. The number of posts.
Content nowadays is not free. A site with a lot of posts has content and adds value to the site.
2. Number of members.
The number of members DOES NOT justify the amount of traffic, but is a good barometer for it. The real key here is to have ACTIVE members. But, even if a member is not active you have their contact information and can be active given the right correspondence.
3. The forums main topic.
A forum that talks about shoehorns isn't going to be as interesting to most as a forum talking about Movies. Since the buyer would probably Admin the site themselves more value is added if it interests the buyer. Also, as we all know, the better adsense value depending on the topic adds more overall value. Better bang for your click buck.
4. The Forums design.
Just about anyone can install a forum and start posting. A unique and attractive design should add a lot to a forums value.
5. The Forums Domain.
A short memorable domain is a must for a forum (imho). I don't regularly visit ComputerHardwareSoftwareAndOtherStuffTalk.com very often.
6. Your Time.
This is so very often overlooked, but cannot be denied. You spent x hours a day putting this site together and running it you should be entitled to something for that. It's to bad the reality doesn't usually justify this.
Well, here is my appraisal of your site!
I really like the design a lot. Also, you have a bit of a twist going to make it unique. The members are a bit low (31), but they seem very active (630 posts).
With the design, the # of posts, membership, domain, and your time I'd appraise this at about $140 Wholesale.
I'd guess your adsense is making about $7/month. My appraisal goes beyond the sites "income".