Reg fee + for reseller market. Low $xx for endusers. Not bad major TLDs all taken. Generally .info extension doesn't fit the forum TLD but once developed it doesn't matter.
I personally think that the best extension for forums is .org
it fits so nicely into the 'community/forum' concept, unless of course you have a commercial forum, like NamePros here
regfee. I personally would not buy a forum domain unless it is in .com or maybe eventually .mobi. its a waste of money. you would just lose traffic to the .com, especially if the .com was also a developed forum.
I think your best bet would be to try and build traffic to build its value. My appraisal is $10-$20 undeveloped but with a few good links and a steady flow of unique visitors this could be a decent forum. Develop, develop, develop.