Well, then that may make it even more expensive to buy. I have a forum that has over 3 million posts and even if it wasn't making any money, I still wouldn't sell it from a rev stand point because of the amount of members and posts it has.
The post to member ratio is not good. Seems like the forum is averaging around 3 - 4 posts per member. (642,294 posts to 146,060 members)
This is a tough apprasial, from not knowing what the niche is or rev it makes, I would say around $12k at least.