Earlier quoted context omitted.
A Dell Precision 7000 will set you back US$3000 at a configuration similar to the iMac Pro. Add a good 27" 5K display, PCI-based SSD and you are close to US$ 6000 with a GPU that's a generation behind. And it comes with Windows. Nobody deserves that.
Wait, but Dell Precision 7000 is a laptop? you can't compare with a desktop. I just built a good gaming PC that can be overclocked including a GTX 1080 for just $2300 (monitor excluded). I'm sure with $4700 more it's possible to get something still better than this new imac pro.. yet with a good cooling system and upgradable at any time. edit: $2300 INCLUDING 15% taxes
You'll probably can get something with higher performance, but it'll probably also take up more space, generate more heat and make more noise. It'll also demand more time to select the components and assemble it. If you enjoy doing it, OK. If you don't, your time is more expensive than the difference.
I selected a Dell because I want a single phone number to call when something goes wrong.