Given the uncertainties involved, your proposed pricing structure seems in the ballpark, though I do agree with other commenters that 1 STEEM may be a bit low and 250 STEEM a bit high.
One use case to consider: I've been looking at chainBB as a possibility for a project involving an established group of futurists. Currently, we use a mishmash of email, loomio, and google docs to keep things somewhat orderly. If file attachments (to posts and forums) were supported as part of some paid tier of a moderated forum, chainBB would be a perfect fit for our needs.
That's an interesting use case. File attachments would be a tricky addition but not impossible. It might be easier to integrate chainBB forums into an existing file hosting platform initially though.
I don't think attachments will be anytime soon but that's definitely getting put on a list somewhere :)
Awesome. File attachments are definitely tricky ... but there is at least one person out there willing to pay a reasonable fee for the service if you ever do decide to integrate them: )