Just avoid the G5s whatsoever. They're enormous, hot, and very poor value for money because they're the fastest Macs that can run Classic, which means they continue to get bid up by people with legacy software. For the same money you can get a Core 2 Duo which will be superior in every way provided you don't need to run MacOS on it.
The problem is that the PPC's with decent performance are ludicrously expensive in small volumes - you can "easily" spend $1000 for relatively anaemic PPC models, so a manufacturer would need to either be able to bet big, or costs are going to be crazy. ARM is much simpler in that respect because of the much wider choice in manufacturers targeting the low end of the market.
Is there any reason why this is necessarily the case? Also if these are being used in embedded applications, I highly doubt it costs $1,000 per unit in quantity.
They're making a kajillion PPC based processors for the Xbox 360, PS3, and Wii, so it's not like they don't have designs for current manufacturing processes. The chip in the 360 can't cost more than $50 today, and if stripped down (2 cores vs. 3), clocked less aggressively (1GHz vs. 3.2GHz) could probably be cut to $25 or less.
Costs more than the Raspberry Pi though.