From what I can see in my country, "Buy it Now" prices for a 'High-Res' 15" PowerBook G4 varies from £95 - £150 depending of specs, condition and any exterior modifications done to the unit!
Bids is a bit more difficult, you have to be lucky to get it quite cheep as the last PB G4 sold for £115 + shipping.
I would personally recommend you get a 'High-Res' model as it is now considered as a "top of a line" notebook when it comes to PowerBooks as it supports DDR2, 128MB VRAM and a 1440x960 resolution (verses 1280x854 on older models). I am personally trying to obtain one as my 'on the go' workstation while I'm in college!
Source: ebay UK for price, everymac.com for specs!