Shhhh! You'll blow my cover! Actually, I just wanted to know because I was considering building my own website, complete with message board. I wanted to get an idea of cost before proceding.
Message boards tend to eat up alot of bandwith. Domain names can cost anywhere between $30-thousands of dollars. And then theres the bandwith issue. All the pictures posted here, threads, articles, etc... It would probably vary each month.
having your own domain can be quite cheap. i got one year for free, but otherwise it is around 10£ with lycos.co.uk. and for 2,5£ per month they will host your domain. But you don't get much bandwith with that. Only 1 GB per month. That's probably not even close enough for a smallish tiny forum.