Microsoft is running a custom SoC that is tailor-made to their platform, with shared memory, power delivery and internal communication channels for the CPU and GPU. This right away reduces a lot of costs you see in a traditional professional desktop, where you have a GPU with separate power requirements, dedicated VRAM, and all the hardware needed to communicate over PCIe on both sides. Not to mention the Xbox One uses really weak (in comparison) Jaguar cores, instead of something much more expensive like Ryzen or Kaby Lake.
The CPU+GPU alone can add up to over $1000 retail on a workstation, and that's a consumer CPU and GPU - once you start adding "workstation" graphics like AMD FirePro or NVidia Quadro territory the GPU can cost over $1000 retail alone for validated drivers to support CAD applications, etc.
Factor in all the extra components that go in to support upgrades (memory sockets, PCIe slots, external connectivity like Thunderbolt) and the fact that these components use much more power than the Xbox One SoC and as such require more cooling and you can see how costs quickly add up.
Now, with all that said - the Mac Pro is pure price gouging. You can get an equivalent workstation from HP for a fraction of the price, but then it doesn't run macOS.
Personally, I find it rather amusing. I remember when the Intel cheese grater Mac Pro was first introduced and Apple was showing off an equivalently specced Dell workstation was more expensive than the Mac Pro. Apple has really lost their way in the professional space (see everything they've done with the MacBook Pro since 2012 as well).