beehaw.org recently posted detailed June financials, and the bottom line is something like $600/month, including 1TB of bandwidth overage, for one of the largest public instances (they’re not responding atm or I would post the link). Before the reddit exodus, lemmy.ml was the largest instance, and it was running a couple thousand users on a $100/month VPS. Lemmy.world has posted itself running on a 32-core/64 thread 128GB RAM dedicated server at hetzner. blog.mastodon.world.
For an instance with only a few users, like friends & family, it should be pretty cheap.