These being a bunch of smaller servers rather than one giant one means upkeep is comparatively cheap. Don't quote me but you could probably spin up your very own instance and federate it with the others for a few thousand bucks. Since theyre privately owned there aren't investors driving for profits, there just needs to be enough money to keep the domain registered and the electricity on.

To your question about gilding comments: As far as i know? No instances do this, and it isn't possible to donate to all of em, just each one separately.

If you would like to donate money to help keep things running, click the sidebar on your main page (not in a comm, the main feddit.org page) and there should be a donation link.