I see four options:
- Server owner assumes all costs of the server. Possible if instance is not huge (ideal on a federated environment) or owner has money and will to do it.
- Accept donations from users
- Require paid subscription to access (and thus becoming private otherwise anyone could still access the server from a free instance)
- Run ads on site. Same as 3, users would migrate.
So I only see 1 and 2 possible