I still know lots that need to work with Classic apps or have been put off buying Intel for now due to one or two OS X apps they use not being compatible, (I know Rosetta should handle this pretty well, but theory and reality sometimes don't mix).
For them rather than investing $1000+ in an iMac, PowerMac or other more expensive machine, they would prefer just to hook a Mini up to an existing set-up and tide themselves over to the point of Intel being okay for them. $500 or less is an ideal price short-term.
While people still want the older PPC machines, prices will remain as high as possible. If demand gets higher, prices may even go above retail. I actually saw a second hand SNES the other day for the same price I paid for mine 12 years ago. While this is an isolated example, I have noticed prices on ebay for these creeping back up. It all comes down to what market there is for something.