I don't think you could get one for 100-300. I know our local mac store buys used clamshells for $250 to turn them around and sell them for quite a bit more I guess. I hear they are somewhat of a collectable and high in demand as far as used macs go.
What are you going to do with the mac? If you are doing anything very processor intensive you will be unhappy with the speed, etc. I would say a used or refurb.iBook (G3 or G4) if you need a laptop.
depending on what you do with it you could go with some sort of external hard drive to store big files if you don't need those files to be mobile with you every time you take your computer.
Mac resellers (local and online) usually sell used macs and might have clamshells.
also remember, the clamshell is somewhat old and you might run into the problem of having something go bad on it (ie hard drive, cd drive, etc.) Apple makes great quality stuff (for the most part) and they usually last longer than their pc counterparts with both hardware quality as well as usability, but things do break as well so you would be taking a risk.