It can depend on configuration (and a bit of bidding luck with eBay). Stock G3s are indeed close to $0. One with upgrades, even just extra RAM, could easily go higher though. Same with the early G4s. Their low point seems to be about $30 right now for stock setups of the early models but some upgrades can send them into the hundreds again.
I generally reference stock configurations when comparing prices on Macs. Calculating value of RAM and upgrades gets too tricky for simpler comparisons.