I think an important part about choosing instances (across the whole fediverse, not just Lemmy) will be to inspect an instance's plan for paying the bills. As you say, this stuff isn't free and even the most enthusiastic admin isn't going to foot the bill for very long on their own.

It seems clear to me now that the traditional way of paying for services via ad revenue doesn't work for the end user. It is open to abuse by those selling the ad space, detrimental to the users, and ultimately doomed to... well, Reddit.

The only other viable alternative is a system based on user donations, with everyone contributing within their means, plus a bit of transparency on the part of the server admins. I've made my peace with having a small amount of money per month being my "fediverse budget", a couple of quid here and there to support the services that I enjoy using. I hope that enough users also come to this realisation too.