Depends on the forum and what you are marketing.
Example I own a web host company. If I put a banner ad on a forum that talks about farming. I am not going to really get anything out of it as 95% of those people have nothing to do with what I am selling.
Now if I put up a banners on a forum that deals with Web Design. I am going to have a lot better shot in landing business. As 95% of those people could use my services or someone else.
Now as far as cost go. That could rang between 50 bucks a month to some sites charge as much as 700 bucks a day.
So you have to do the math and figure out which sites work best for you for budget, views, and landing new customers. I try to use the triple rule. If I spend 100 bucks I need to make back 300 off it or its not worth my time.
Hope this helps