Well, an entirely non-profit -- at this point, in fact, unincorporated, with no assets to speak of at all -- writing group that I'm part of migrated their forum in 2019 from a PHP forum package (I think SMF, although I wouldn't swear to that) to Discourse, and it runs just fine on what I'm fairly sure is a $10/month Digital Ocean droplet.

I don't doubt that Discourse requires more resources than most PHP forum packages do, but it doesn't need that much more.