Just a bit of a rant on my part, but the Mac Pro is a terrible computer. Well, not unless all you use is Final Cut Pro X and have the dual D700 option, but in every other aspect it's over priced and underpowered just to get a nice form-factor, and even that's not really an advantage anymore.
A fully spec'ed out Mac Pro is $9,600 as per the Apple configure tool, here is what you get:
E5 2697 V2 12-Core @ 2.7Ghz (3.5 turbo)
64GB 1866 RAM
1TB PCI-E SSD (Samsung XP941 with double capacity)
Dual D700s (lower clocked Fire Pro W9000 analog)
My user-built configuration focuses on minimum size - the case is 1.6x the volume of the Mac Pro. not too shabby.
NCASE M1 - nice and stylish vs trash bin.
ASRock X99E-ITX/ac - the build enabler.
H100i cooler - water cooling, yeah!
E5 2699 V3 18-Core @ 2.3Ghz (3.6 turbo) - 6 more cores, and always faster than the 2697 V2, even when all cores are running. Or swap for 5960X and do 4Ghz all day 'ery day.
64GB 2133 RAM (2x32 sticks)
1.2TB Intel 750 (2.5" version via M.2 adapter) - way faster than the Mac Pro SSD and a free 200GB.
GTX Titan X - 2~3x faster than a single W9000, so even the dual D700s are outclassed by this one card, except possibly for FP64 performance.
Silverstone 600W SFX PSU - They recently came out with a 700W unit as well.
Slim Bluray/DVD drive -
Grand total? $7,900 for a computer that'll rip the Mac pro a new one. Go 5960X and you're down to $4,900 and still better. You can't even spend that much money in PC land if you wanted to.