Usually I would estimate that a PC looses 50 % of it's value every year. But with Macs I think the figure is somewhere around 30 % judging by the prices sellers demand.
Certainly build quality and no malware goes a long way, but my 1 Ghz iBook wasn't exactly lightning fast, so I would be very wary og buying anything with less than 1 Ghz G4. At which point the prices should take a bump down. But they don't.