Mac cards differ in that:
They offer ADC ports. This addes some compexity as the video card has to route power to a connector.
The ROM is typicially bigger for a Mac system for some reason.
Newer cards are AGP Pro for the G5 system, lacking the extra power cable inputs the PC versions have.
Basicially they cost more to recoup the additional development time for a product that goes to a much smaller market, and usually has different features geared to a platform that actually advances.