Ross, forums must be one of the most difficult things to sell. They require too much work, so only a person that is 100% interested in your topic will be interested in buying it. That narrows the market a lot.
PR isn't important anymore. Those days are gone since Google stopped doing the regular PR updates. No serious webmaster cares much about PR anymore.
50 or 60 uniques per day are really nothing. How could you get some income from such tiny traffic? No one would pay hundreds of dollars for a site if there is no possible way for it to make some income. And don't try the "potential" pitch, since that isn't a good point. People all over the place try to sell sites and forums saying things like "it has huge potential" "in the right hands it could make thousands" and similar stuff. Real income, real traffic is what matters, potential is just a dream.
You may need to hang on to the forum and make it grow 10 times more, and then maybe you would find a buyer, but as it is now, I must agree with everyone else and only give it a value of 2 digits, maybe a couple of hundred bucks if you are lucky.