Code:$1200 Xeon W-3225 $500 Supermicro X11SPA-TF $275 4x8GB DDR4 ECC RDIMM $200 Power Supply $175 Radeon 580 $50 EATX Case $35 M.2 256GB SSD
About $2500.
I actually priced out the workstation components:
CPU - Xeon W-3223: ~750$ (https://en.wikichip.org/wiki/intel/xeon_w/w-3223)
Motherboard - C621 AORUS XTREME: 1800$ (https://www.bhphotovideo.com/c/product/1460397-REG/asus_rog_dominus_extreme_eeb_atx.html/?ap=y&gclid=Cj0KCQjwrdjnBRDXARIsAEcE5YmUxq_U_KtEUSG5uox1vEkJ39AyNlbTS0_-cEc-1VFQtPcIkGa3PTcaAtiLEALw_wcB&lsft=BI:514&smp=Y)
RAM - 4x Supermicro 8GB 288-Pin DDR4 2666: 296$ (https://store.supermicro.com/memory/ddr4/8gb-ddr4-2666-mem-dr480l-hl02-er26.html)
Power Supply - Enermax MaxTytan 80+ Titanium certified Full Modular 1250W: 400$ (https://www.newegg.com/enermax-maxtytan-edt1250ewt-1250w/p/N82E16817194132)
GPU - Radeon Pro WX 7100: ~ 500$ (https://www.newegg.com/p/N82E16814105069?Description=radeon pro wx&cm_re=radeon_pro_wx-_-14-105-069-_-Product) - Its the closest thing i could find to the Radeon Pro 580X
SSD - SAMSUNG 970 EVO M.2 2280 250GB: 135$ (https://www.newegg.com/samsung-970-evo-250gb/p/N82E16820147689)
Dual 10G NIC - AddOn - Network Upgrades 656596-B21-AOK Gigabit Ethernet Card 10Gbps PCI-Express 2 x RJ45: 440$ (https://www.newegg.com/addon-network-upgrades-656596-b21-aok/p/N82E16833516137)
---------------------------------------------------------------------------------------------------------
Total: ~ 4,321$ Note: This does not include the case, multiple TB3 ports or an operating system.
Base Mac Pro: 6,000$
Yes, its expensive, but given the workstation grade components, i do not feel that it is over the top expensive. More research is required, but i would hazard a guess that it may get more economical as the SKU goes up.