Search for your model (including speed: e.g. 1, 1.33. or 1.5 ghz) on eBay. Then check the box in the left margin to: "Show only: Completed listings". That will give you recent completed auctions for your model. For comparison, use auctions as similar as possible to yours in features (RAM, optical drive, hard drive), condition, and Applecare warranty.
e.g. searching for PowerBook G4 12 1.33 (my model), recent completed eBay auction prices for good condition models hover around $700, plus or minus--depending on features.
That said, it also depends on where you sell it. eBay prices for popular items like PowerBooks tend to be higher than for the same item on Craigs list or in your local classifieds.