I think $400 or so. Anyway, keep in mind that the G4 takes more expensive RAM and can handle less of it. That's a real negative, because with a slow CPU you need lots of RAM.
FYI used and refurb minis are a little tough to get now. I think they're very popular systems.
I wouldn't buy a desktop system now anyway until Apple updates the current line of iMacs and until they clear up the future of the mini as a product.