Main reason as I see it for the extra cost on Mac Cards is the fact that the firmware has to be custom written as Macs don't work with the PC style BIOS on your normal PC graphics card. Sun SPARC hardware uses some of the same graphics cards as the PowerPC macs and again they are more expensive. Weather you believe the high cost is in proportion to the extra development cost for different firmware for a lower number of sold units is debatable though!
I break stuff.