[Lemmy] servers can be run for only a few hundred dollars per month.
The median seems to be much lower, like 10, 20 or 30 per month. Many admins reported they ran a server for other purposes anyways, and just had to pay for the domain to add a Lemmy instance.
It’s only after a few thousand users that the bill goes 3 digits.
At least that’s the impression I got from reading a few posts about this.