Good answers here, but to summarize some points and elaborate on others:

• Ultimately it's the admins that choose to stand up an instance who are responsible for that instance's costs.
• The costs are well within what many hobbiests can afford, so some admins aren't asking for help, but many instances have donation links in the sidebar or pinned posts.
• The developers are also self-funded, and for many it's a full time job; they also have donation links, and I personally think this should be a high priority for people who are able to contribute and want to see Lemmy grow.
• Yes, a well-funded outfit/person can stand up a big instance with more resources than others, with intent to somehow turn it into a business (e.g, selling ads). That's fine; no one has to go there if it's not worth it, and other instances can defederate if they cause actual problems.

I did a one-time donation to my instance admin and another to the developers for now. If I stick with this, like I think I will, I'll probably do some monthly thing.