Z8s can be configured >$100k. Also those have fewer usable PCIe slots. Dollar for dollar I'm not sure the HP or Dell workstations are much cheaper than the Mac Pro if at all. They seem like just a different spin on some standard Intel configurations, with build quality good but not like Apple's.
3TB RAM 56 cores 48TB
No word on cost, but the entry level goes down to below $3k.