Frankly, I don’t think the idea is to make money with the fediverse. However the idea of balancing your costs is good for longevity of your instance. You can try opening a liberapay, opencollective (etc.) for donations. You can also try offering more for money, for Mastodon maybe those custom emojies ? I don’t know. Or the original WhatsApp model where you get a year free and then have to pay a dollar per year.

I think the lack of ads/paid boosts or something is probably why people join mastodon. I guess if you really want to go that way you could offer to make all new accounts follow someone’s account.