All costs solely depend on you and what you're looking for. For just a forum you'd probably only need the least costly hosting package from whichever host you choose, unless you choose to be your own host or know someone willing to host for you, like Mo apparently will.
Domains are extremely cheap unless you really want one that has already been secured by some company looking to make a large profit by selling it. Usually you can find domains costing between $1-8 annually.
Also, the forum software itself may have certain costs, if you're looking for certain software in particular that just so happens to be costly. IPB, for instance, is $150, or $300 if you're interested in the extras. Otherwise, there is plenty of free software out there for forums that you wouldn't have to purchase or otherwise pirate to use.
I have personally found
1&1 to be an intriguing host, and had planned to go to them for domain name and hosting if I ever wanted to make a website. I've had this site bookmarked for quite awhile now.
Each host you find is going to have their own perks, it's really up to you what you want. Homestead, for instance, has it's own sitebuilder. A regular and lite version. Regular, for working in depth at home, or lite for making some changes while away from your personal computer. Some other hosts offer their own builder as well, and there's also software such as Microsoft FrontPage (I believe it's called Web Expression now though) which can help you build your site. (These are usually unnecessary when it comes to forums since the software for the forums itself is usually all you need to have if that is all that's going to be on your site.)
Since it sounds like Mo is going to help you out, it is probably irrelevant, but I figured I would give this info anyway, just in case you find some use for it.
