It's entirely Brexit.
This day last year, £1 was worth $1.527.
Today, £1 is worth $1.223.
(1.223-1.527)/1.527 = -20% almost exactly, and the Mac Pro has increased in price by 20%.
Since GBP is fluctuating a lot, I wouldn't be surprised if Apple's prices include some extra leeway in case of further swings.
(And aren't I glad I emigrated from the UK last year...)
The newest Mac Pro used a Bloomfield CPU (2008) or a Westmere one (2010) and a circa 2009 GPU.
Installing OS X took only a little longer than on my native Macs. I just had to tick a few checkmarks in the Hackintosh post-install software to customize for my setup.
OS X 10.11 updates I could install as soon as they were available from Apple. No problems at all. (I of course checked the appropriate forums for compatibility notes but there never were any problems reported). Haven't installed 10.12 yet (as I'm not too thrilled by the OS on my MBP anyways) but I heard it works just fine on Hackintoshes.
Now the caveats: I disabled the onboard audio and attached external USB speakers instead. Because onboard audio seems to be more broken than not and USB audio "just works". I'm also connecting the machine via ethernet and don't have a WiFi adapter attached. Oh, and I got lucky in the first place by choosing OS X compatible hardware (Intel CPU, ASRock mainboard, AMD GPU).
Things that doen't work: Browser DRM with Safari. I can't watch Netflix on the machine. iMessage support is cumbersome. And I can't submit binaries to the app store from the machine.
But otherwise for work it's a beast (compile times are great compared to my MBP) :)
As long as you use Intel CPU/AMD GPU you're golden. Intel's HD GPUs work, too. Nvidia GPUs need 3rd party drivers from Nvidia.
edit: the key to a good experience seemed to be finding a working configuration then leaving it alone. If you want to update the OS regularly, you'll probably be unhappy.
It's even worse than that, if you're in the UK: the price of the 2013 Mac Pro has gone up by £500. Yes yes, Brexit and currency adjustment, and if it were new hardware I'd clench my teeth and bear it. But a £500 increase for hardware that's almost three years old is highway robbery.