You can get it cheaper if you self-host, but it still requires a VPS, which, on average, costs more than most are going to want to pay to host a forum.
And with VPS you have to maintain the actual server. You can't just install it and be done. If you aren't already familiar with setting up servers, handling access, setting up software, and setting up DNS it will take you evening or two just to get going and after that it requires you continuous maintenance and how many of these custom forums actually have backups in case something goes wrong?
