There's an almost $25k difference in price in the US between the 550 and the 650GC. I'd expect that difference to carry over to the M models. That's hardly a rounding error for most people.
MY13 Sakhir Orange M5 6MT
Retired - MY08 E60 550i 6MT / MY04 E60 545i 6MT / MY98 E39 540i 6MT / MY94 E34 530i 5MT / MY84 E28 528e 5MT