I know bloggers who make an entire living from Adsense, but they really have to work at it, blogging on topics that have good ad linking. Much harder to do that well with forums, I think. I've chosen not to run ads on forums, but have received very good donation rates.
I experimented with Adsense on the thousands of pages I have that are not forums or themselves selling retail product, and eventually reduced the ads to just three pages that performed well. Originally two, but expanded to three to increase ad exposure. A grand total of $1388.44 since late 2004
One thing that was mentioned here is that advertising rates are all over the place. Visit some sites and start looking for links about advertising and you start to discover some details. I have quickly noticed that the more traffic a site has the more it costs to place ads/banners. I also noticed that web sites with good content attract advertisers.
Adsense is okay but it seems sort of hit or miss. I like having specific sponsors especially if they offer my visitors and members a really good product or service benefiting everyone involved. If you run just a forum I don't see much potential unless the forum is very popular with a marketable theme.
Snooping around has also returned my thinking about flat fees (price/month, quarter, year) and ECPM (effective revenue per thousand impressions) There is a lot to think about but I think with a little effort most decent sites could earn you some money. I have been using OpenX for by banner serving and the more I use it the more I discover how powerful it is and it is FREE! So, Once again I sort of answered my own question - the Open X software has a lot of good help files and OpenX.org has quite a bit of information that explains all this.
