Especially how Lemmy is right now, only a small portion of users would be needed to sustainable keep an instance running. Maybe from every 1.000 users, only 1 would be willing to pay 10$ a month and it should be more than enough.
Shit changes quickly when somebody thinks it would be a good way to start allowing video-uploads. It can get expensive fast with that amount of storage and bandwidth needed. I can see instances selling small "premium" subscriptions for videouploads. You could still host your own instance and get videouploads completely free for yourself, but if you don't wanna go that route, it would make sense (and would be totally fair)