well i am the owner of a forum, and it is a long and hard road. I had no clue what i was getting my self into. Unlike other forum owners, i have no time. AS of now, i am just losing money
I remember one of our forums generated a signup from an ad we had up within hours of launch, but yeah, they can be tough to make money with in the growth stage.
About a year ago I ran a forum and had $100 in donations in under a month, although it was a forum for somebody els, appox 1000 members in a week, all active, so yeah, if it's big, then yeah you can make a lot, plus if you have like VIP content etc.