Last revision Powerbooks, maybe $700-$1000, depending on condition. I paid $1100 for mine because I bought it through Apple's refurb store, but ideally you'd be looking at around $800 on Craigslist or something.
I love my 12in PB and I love the 12in. ibooks as well. I think people may desire them because you can't get anything smaller than a 13in for portability. On the otherhand, if they came out with a 12in MBP..the value would decrease significantly. But as for now I know I wouldn't trade my 12in. PB for anything less than a grand...thats just my opinion.
I'm selling my 12" ibook G4 1.33 ghz to someone this week for $700. It's in great shape (no significant physical damage, no dead pixels), has the RAM maxed to 1.5GB, has 2 years left on Applecare, and comes with an Incase sleeve, Radtech Screensaver, and all of the original packaging. I think that both myself and the buyer are getting a good deal.