It's also possible to outright buy Mac Pro Rack, a $6500 full depth 4U server, that is 1CPU all the way up, that offer neither any redundancy nor any remote management, if that kind of trash matters at all.
The lack of dual power supplies and remote power on/off and good PXE support is crazy for a rack mount computer (won’t even dignify it by calling it a server).
The rack mounted Mac Pro isn’t targeted at the server market, it’s targeted at creative industries (i.e, film/tv production, audio recording, etc) where you’d have it rackmounted into a flight case as part of a mobile post-production unit.