I've seen them anywhere from 150 to 650 depending on condition, and upgrades and who happens to be bidding etc. If you are patient you can pick one up on the cheaper side.
Have a look at the sold items in eBay auctions (see the link below).
Macuserman's estimation is not at all bad. The range for PowerBook G3s on eBay US is anywhere from $50 up to over $600 for a Pismo with a G4 upgrade.
I personally would take $200 as a rough sort of benchmark, depending on how badly and quickly you want one.
I would think a Kanga would command a little more than the Wallstreet and "newer" generation g3 Powerbooks. Its an interesting model you dont often see.