But if you can build a box yourself? You can save gobs if money over the Mac Pro. Gobs. Or become a male prostitute as one poster suggested.
You don't save gobs (When I was young, I loved building systems - nowadays, my time is valuable)- you save a little bit, but I'd rather pay someone else to do all of the testing and deal with any hiccups.
Go with an AMD based system and you will get better performance for a lot less money. The Epyc CPUs are very interesting.