I do rendering on my 2012 12-Core Mac Pro. It would cost me almost three times what I paid for that to get a new 12-Core Mac Pro to replace it. Which, thanks to my software moving to CUDA, would make a new Mac Pro useless to me. Add to this the fact that no modern Mac supports CUDA due to Apple going with AMD GPUs, and that pool of software is growing ever smaller. I'm almost to the point of abandoning Macs altogether and getting a Windows multi-GPU workstation instead. For a hell of a lot less than a new Mac Pro.
Unless every vendor out there suddenly decides to support the trainwreck that is OpenCL, I've long ago purchased my last Mac.