Absolutely. In Mastodon it's a pretty simply setting IIRC and I expect it's commonplace across other microblogging platforms that use ActivityPub. I certainly wouldn't be against the principle of joining an instance that was paid for.

The whole "millions of users" fallacy is the result of people not being able to grasp what federation is about. The network can easily accommodate millions of users. Individual instances don't need to be able to.