On the upside, atproto "instances" (we often call them "personal data servers") have much less responsibility than the typical Mastodon instance. So folks should find them quite cheap and easy to run. One reason is that they don't need to directly support application features such as timeline construction, search, image optimization, etc: they just host your social data.
We have 10 servers federating in production, each housing around 270,000 users, and it all runs on disk and sqlite, quite affordable! We migrated 2 million users off a single large host onto these 10 smaller ones transparently and without need for much fanfare. We're dedicated to the tech and seeing it work in production has been super heartening.
