Actually I’ve set up quite a few websites for various people including others that host forums.
The forum software itself is the majority of the cost. It’s $180 USD for the software + one year of upgrades and an additional $60 each year after that for upgrades. (which still isn’t all that much when you think about it).
The .org domain name is never really more than ~$15 a year unless you’re going with some ridiculous registrar.
Also, this forum is hosted through DreamHost (www.dreamhost.com). Their services are extremely cheap. So unless whoever currently owns this site decided to go all-out and get some heavy duty (unnecessary for the amount of traffic on this website) dedicated server through DreamHost, they’re definitely not paying more than $20 a month for the hosting.