I think it's all depends on the traffic for the forum. If you have large number of people coming to the forum every day, than the more money you'll make. It doesn't really matter whether it's a forum, blog, website, etc. It's all about the traffic. The more people you can attract, the more money you're going to make. But the good thing forum is you can build a loyal fan base who will keep coming to your forum to discuss or promote their products/services.
For the most parts adsense doesn't convert well with forums. A lot of click are worth next to nothing(CTR is low) and you don't get many click per 1000 Unique visitors. Of course it depends on your niche, but people are too used to forums and advertisement layouts of them so they know not to click.
Ok thanks for the info. I've ran a few forums before but never really thought about monetizing it so was just wondering.