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.
Current - 2014 911 C4S Cab 7MT | 2012 S4 6MT
Ordered - 2015 911 C4S Cab 7MT
Retired - 2013 F10 M5 6MT | 2008 E60 550i 6MT | 2004 E60 545i 6MT | 1998 E39 540i 6MT | 1994 E34 530i 5MT | 1984 E28 528e 5MT | 2008 911 Targa 4S 6MT | 2000 911 Targa 4S 6MT | and many more . . .