I personally like the clamshell iBooks... I paid about $630 for my Blueberry back in 2002. I still have it, though I modded it to remove the white metal shielding because it was collecting bits of black dust behind the LCD. It looks awesome now.
I paid $200 for my pristine Tangerine iBook last year, as it came with its original disks, AirPort, an external USB hard drive, and a boxed VST Zip drive. Its original 3GB hard drive needed replaced (because it was waaay too small to be usable), but I upgraded to a new 16GB drive in about half an hour (I've had those things apart more than once).
They're both awesome machines. The only issue I have with them is that they're only 800x600, which is kind of cramped, particularly under OS X. They're built like tanks otherwise, and I still love the handle.
Anyways, the first gen models (Blueberry and Tangerine) are fairly common, so they're pretty inexpensive. The 2nd gen is a little faster, and added Indigo. The final versions had FireWire, DVDROM drives, faster processors, and faster GPUs. Plus, some of them (particularly the Key Lime) were fairly limited, and thus are still a little expensive, at least when in good shape.
I'd say between $100 and 150 for an original, $150 and $200 for the 2nd version, and up to $350 or so for a Key Lime clamshell in good shape. Obviously, condition and accessories dictate prices. If they want $150 for a stripped original Blueberry with a dead battery, covered in scratches, without even a power adapter, I'd walk away. The power adapters alone (Apple originals, anyway) are at least $20 used.
