My vBulletin board uses between 4 and 6GB per month, I have about 145 members, 4,000 threads, 40,000 posts, and my database is about 70MB. Nothing is on the main site at the moment, so it is really only the board using the bandwidth.
... I don't think that BB's are hogs. Let me explain why:
Let's take f.e. vB. If you (as on WHT) disable all the Avatar and Signature crap and empty "searchindex" and that useless "word" table and make good use of gzip then you can really minimize your bandwith. Also you can set LOAD LIMITS in the CP so the vB reacts when server is in high CPU load. I had a RaQ 4i once (K6II CPU, like old Pentium - 512 RAM - 15GB HDD) and it handled around 12 vB's with ~2-3.000 members each and a total of 400-500 MySQL connections.
The trick is to not use "pconnect" for example and tune the mysql conf to use a small timeout value for example and not use much memory for a MySQL call. Reason is that a message board is someone online for 90% idle (reading stuff) and then posting something. So perm connections are pretty useless a huge memory frame too as ther is never much data, I mean how much does a guy type in a post right?
Over 98% of BB's are not correctly managed or not at all! THAT is the problem!!!
Message boards are basically just a bunch of text, so unless your message board is very popular it should not consume any considerable amount of bandwidth at all.
For example in my experience a relatively busy message board which often sees 10-20 simultaneus users and many new posts a day might only need 1-2 GB of bandwidth a month, or less.
That said, if you have a lot of heavy downloads like custom avatars that would obviously use more bandwidth. The only concern about message boards is CPU usage, and with some of the newer boards such as vBulletin and phpBB even this is relatively well managed.
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My forum (wbb 1.1) is just short of 20,000 posts and has been online for 5 months. It uses on average about 2.5gb a month, but then again i have avatars and sigs enabled. It only uses up about 30mb space, not including the 4.5mb database.
But it does take the server load high at peak times, I think i need to do some of what Sharefile suggested