Simplest answer -- spend as much as you are comfortable with. There really is no bad answer for audio with any of the new Mac Pros. The graphics power is of no audio consequence for now (nice if you do audio with video) and any purchase based on what may get added to take advantage of these is a pretty poor bet in my opinion and by the time it matters there will be better solutions out there...
I posted this elsewhere but here are my general thoughts on new macs and audio... My signature shows my personal solution.
One doesn't buy speed anymore (all machines below Turbo up to within less than 10% of one another) - you buy form factor, maximum parallel processing power and noise performance...
http://www.marco.org/2013/11/26/new-mac-pro-cpus
MacMini/MBP i7 quad TDP 45W-47W: These two machines will Turbo up to 3.6GHz - 3.8GHz but once the 90deg CPU point is reached fans will ramp up and they will fall back a lot to 2.6GHz. On the 2012 MacMini that was about 25% constant CPU load.
imac 27" 2013 i7 Quad: TDP 84W: Turbo up to at least 3.7GHz (3.9GHz for single core tasks) and due to higher TDP and much better heatsinking will stay up there till 70% load or more before ramping fans and falling back to 3.5GHz. (very similar resullt for 2013 imac i5) - note: 21.5" 2013 imac uses 64W TDP CPUs with subsequent reduction in threshold to ramping fans and loss of Turbo boost).
nMP Quad: 130W TDP - Will turbo up to at least 3.7GHz (3.9GHz for single core tasks) and due to the highest TDP and best heatsinking should just stay there. CPU performance near identical to imac i7 but much more Graphics power, faster RAM and faster internal SSD.
nMP Hex: 130W TDP - Will Turbo up to at least 3.6GHz or more and at worst come down under very heavy load to 3.5GHz - literally 50% more CPU power than Quad.... and so on to the 8 core. For the 12 core you better need 12 cores otherwise it will be slower and pretty much a waste...
The much more important question nowadays really needs to be "how many cores do you need" and "how quiet do you need it to be under a demanding load"? For my use - Pro Audio) my 2013 i7 imac can do hundreds of tracks and hundreds of plugins with CPU temps in the mid 60s degC- extra fan noise above base = zero.... I have yet to stress it past 30% in real use... very quiet machine!