Cheap hosted forum software?

I have a B/S/T website that I'd like to add a forum to since the main application doesn't facilitate discussion. I'd like to be able to set up categories for discussion instead of one large topic/response list. I'd like it to be hosted since this isn't my main application I don't want to waste time with updates, maintainance, etc. Plus I'd like it to be fairly cheap, NodeBB is $250/mo and Discourse is $100/mo which are currently pretty far out of budget. Is there a cheap simple solution?