DOUBLE the cost? Wow, I would never have guessed that much of a cost difference in a million years.
I'min the same general area as you, and I would say it's even higher, at 2-3 times the cost for std. grey, broomed concrete. It's just not nearly as common here as in the Southern states........
Oh ya because you need a high percentage of cement plus it's needs to be a pea gravel mix not including you'll need double the man power if doing larger areas.
We actually use regular (3/4") low-chert stone for stamped, just like any other exterior flatwork, and tend to pull back on Portland cement and replace it with flyash to minimize the potential for bleeding with all of our stamped work.
I agree on the manpower though, and you don't just need "bodies" to stamp large areas, you need guy's with experience that can move proficiently. I tend to try to sway most people away from patterned stamping, as shown in the second & third pic, and towards seamless stamping, like the first pic, because of the extra labor and complexity involved.