As always lots of variation on price. Last year a local fellow we deal with was selling off some G5 1.8DP, 160GB, SD, 512mb ram off lease machines for $600 cash cdn no taxes.
I would always check your local shops, in our case Beam Echo which I usually use as my high price yardstick,
BeamEcho | Pricelists - Used Apple Hardware
MacDoc who is usually lower priced than Beam Echo,
the classifieds here, craigslist and kijiji as well as doing a local search on eBay.