hosting 15,000 users a month is trivial for a forum use case. building a forum is also trivial. you can literally do the entire thing in 15 minutes (buy a domain, use cPanel, install phpBB, done). I think 5K a month is more than appropriate.

you're greatly overestimating the difficulty of setting up a forum and the costs to have 15,000 active accounts with 1 hour viewing time.