There are additional costs that aren't factored in (although they're not specifically lemmy, and I pay for them myself rather than use donations) such as a very cheap vps for our status.lemmy.zip page and i got a 3 year deal for an external email provider for less than some email providers wanted a month.
Our server is the same one feddit.uk use (a hetzner auction server) although lemmy.zips I think was a touch more expensive per month. But it's got a lot of room for growth.
Also we now host all our backups offside too, which adds a little on top. I'll probably cover this in the next server update.
Ibis is really early days. I hosted it for a bit and had things like federation issues and softlocks. Will be really interesting when it's a bit more mature though.