Have they said what the cost is going to be? The most expensive part of the Mac Pros has always been the CPU, which would be the same cost as if you put that CPU in a windows "gaming" machine. For example, the "current" Mac Pros have 2 Xeon X5670 CPUs... which run about $1,400 EACH... so right out of the gate you have $2,800 for the dual CPUs it uses.
Basically they are not gaming machines... they are using server-class CPUs.
So you are comparing apples to oranges unless your gaming machines are using the same CPUs at least.
I just had to put E5-2680 x16 CPUs into 8 servers, but yeah... I'm sure I could have gotten 32 complete gaming machines just for the price I paid for the 16 CPUs... but they aren't for gaming.