Let's take a break, the analysis of server costs is boring and I don't like doing it, and I'm sure people will throw numbers at me of the absolute race-to-the-bottom hosting numbers they can find to store and run all this stuff, but really that's not interesting to me.
Let's do a comparison.
Remember that the idea of "fully self-hosting" on Bluesky/ATProto at this point is primarily abstract; nobody is really doing it. But of course there's a place where tens of thousands of people are running their own servers for millions of users, and that's the fediverse/ActivityPub.