Alrighty. Let's start with your Power Supply... the Mac Pro is running a 1.5KW PS. Are you accounting for that? Your $50 case is going to compare to a custom designed Aluminum enclosure for longevity and thermal efficiency? Where is your fan, cable, and cooling budget? How loud will this machine be?
Plus your spec'd MB only supports a 205W CPU, not the 300W plus that the Mac Pro offers. And you need to add in a warranty of some sort (intangible benefit), and all of the custom controllers (T2, TB3, etc).
Sure, you can get the entry level performance at a cheaper price, and a DYI guy can go build their own hardware solution and sideload macOS on it. But if you're running a production studio and have employees that draw salary regardless if their computer is working (and don't have time to set up and maintain some custom rigs), then you buy a Mac Pro and forget about it. And don't discount all of the time it takes you to build and configure the computer, which is ~$50/hr minimum for an insured contractor.