Well, an older G3 at 500Mhz should run about $350 - $400. An 800 - 900 Mhz will probably run from $500 - $600. That's for the G3's. For a G4, I'd just buy it new, especially if you can get Education pricing, 'cause they're only $899.
Oh, these prices are all relative, of course, and I got them from eBay. I got a 600 Mhz G3 a few months back for $450, so you know. It's all relative, but they should fall in those price ranges.