You can't really compare that build to a mac pro. On the low end, you're mostly paying for the platform:
- Full-sized, machined metal case
- *Huge*, complex, dual-socket Xeon motherboard
- 12 memory slots, 6 memory channels
- "MPX" slots
- Custom video acceleration card support
- Dual 10 GBit Ethernet, Thunderbolt
- A lot more PCI-e lanes
- 1.4 kW power supply + cooling capacity