That's something I really can't figure regarding space and forums. Of course forums takes up space, but not as much as running a file distribution site.
If I were to choose whether to run a Flash learning tutorial site or distribute Flash movies, Flash tutorials would be better in that ads can be placed on the page. If each page was an avg of 30k, then I'm insured that for each 30k, 1 ad is display.
If the site was based on files, then each file has to be at max, 30k to be able to generate 1 ad display.
I don't think the problem lies in the amount of space a forum uses, it's more rather the CPU usage and bandwidth. Proboards for example has added 3-4 new servers since the 1st of this year.
A 17 GB SCUSI drive runs about 250 these days I believe, but that's a one time fee.
Bulletin boards are easy on the bandwidth if you server can compress the pages, however I think that most free remotely hosted scripts don't know the technology that well or run it in the proper manner to keep it up.
The company I work for, each Gig of bandwidth costs $1.23 cents for reliable connectivity. If I were to go out and buy a 1U server for around 1500 bucks, then the only thing left to pay for is space for co-location and bandwidth.
Hopefully, I'll figure a way to run banners that don't annoy the heck out of people (I hate popups), but can pay for the server hosting.