If you purchase Mac Pro 2010 or 2012 with two processors (2x6=12 cores), I suggest you to upgrade its CPU into Intel Xeon X5690 (3.46GHz),. Till I write down this today, the price of X5690 on ebay is around 400$ for a pair (two) of CPU alone or, around 600$ for a pair of CPU plus kits. This price is not included the service. You can find "how to" instructions on YouTube or other place.
You can buy the lowest Mac Pro with 2.4GHz (around 1000-1500 now ) then upgrade its CPU by your self. Don't forget to upgrade its RAM too!
But, be careful to choose the right one since many (!) Mac Pro 5.1 is actually the upgraded Mac Pro 2009 (4.1).