The 300 - 500 dollar range consumer cards seem to crush all over the $4,000 cards for anything I might use. Are there popular apps that are locked to the Pro series cards or something? Vegas rendering, Autocad, Folding@Home, Unity Dev, etc.
Why would I want the >$3,000 D700 in the Mac over a 290x, even if the 290x wasn't 1/6th the price, since the 290x is faster? What's the market for these cards?
What are you using the GPU for? An actual Vegas, Maya, or proprietary in-house rendering engine user could easily answer your question–
• Enough VRAM to load their entire dataset on the GPU (yes, the Mac Pro skimps on this)
• Hardware optimized for pro-level GPGPU (e.g. ECC RAM, but the Mac Pro skimps on this)
• Dedicated pro-level support from AMD and a pro-level expectation of QA before you buy it; think of those proprietary in-house tools here (you're not likely to get AMD support for the D300 since it is custom to the Mac Pro)
If none of those seem that important to you (you're just mining altcoins or playing BF4), you really _don't_ benefit from a $4,000 card. AMD knows that.
For the guys that need it every day to get their work done, $4,000 is a pittance for what they get in return.
Obviously, I concluded that the Mac Pro GPUs fail in all respects to compete at the pro level.