Generally, I make more money on text advertisement sales than from banner slots. Webpages are EXTREMELY higher in terms of CTR etc because the majority of forum users are return users. Most are educated on where to click on the forum and many stray away from the ads. People involved in forums are often technologically savvy (my site is an official gaming forum for a popular MMOCC). We used to sell premium memberships but the game started prohibiting us from doing that. I didn't make a ton of money from the memberships, made more for the ads, but the text ads did the best. Sold a few for $8 or $10 a month. I tried YPN with little results. I've had the most success with Google. I actually think payment is decent for the amount of clicks I am getting which is not much at all. Forums are hard to get clicks.
Forum Management is one of the toughest jobs that i have found! I'm into the webmaster arena from last 5 years and making money through forum hasn't been an easy job for me
I also heard from several people that it's very hard to get them of the ground. Think of it this way: would you register and post on a forum with very little posts and activity?
Forums are tough. They cost more to maintain due to the server load, and the traffic is worth far less. You may be able to convince some businesses to do direct media buys, but good chance the ROI won't be there for them and they'll cancel.
The best way to make money from forums is to establish them, get traffic and get it running and active with users, and then sell. Kind of Forum Flipping. There are always people ready to buy established forums. I haven't done it, but i had a friend who made a good amount of money doing this. He sold his forums to his usual buyer folks on yahoo or msn, who later did some tweaking and sold it to other major buyers, i have no idea where, perhaps in here or sitepoint.
Forums are a lot harder to monetize, because the visitors are there for the forums, not the information (unlike blogs). On the other hand, some forums are a lot easier to monetize than others. Webmaster forums are the hardest because webmasters are ad blind. I run a swimming forum, and I have both AdSense ads, Inline ads, and affiliate ads to one of the biggest swimming gear shops on the internet - I make a lot more from that than my proxy webmaster forum.
In my opinion forums are way too hard too monetize. is better to start some sort article based websites