The G3 is going to be cheaper obviously, but go for a G4 anyways. Depending on the specs, a G4 could be anywhere from like $300-$800. The $300 is a guess because I'm not sure what the specs of the earliest G4 were.
Check the prices on eBay for completed auctions for the iBook G3 and iBook G4 to give you an idea of what they are going for. I would say that an iBook G3 (clamshell) probably isn't going to cut it. You're best price point would be a used iBook G4. I'm assuming that they would be down significantly in price with the introduction of the new MacBooks. The referbished 14" iBook from Apple is going for $999 which I think is kind of high considering a 13" MacBook can be had from Amazon for $999 and it's twice the machine.
eBay 14" iBooks are going for ~ $750.
I didn't look at the older iBook G4s. If you are considering an iBook watch out for spending $500 for one then having to add more RAM and a new battery and a copy of iLife and... It would almost be worth it in the long run to buy a new machine.
Ya, but the key is, to wait just long enough. Like with Cars. Cars from 92'-97' are dirt cheap. Becuase they're not new enough to be pricy, but not old enough to where they become vintage. same with records, cassettes and Cd's? Guess which cost the least? The one in the middle. Im thinking G3's are just at that transistinoing point to where they become vintage, and G4's become the least expensive