Good luck.
I don't think you will be able to find one that cheap, unless it is an
old iBook from eBay. If it's just for school work like typing papers and surfing the web, then an iBook G3 or G4 will be fine. You can probably find a iBook G3 with a 1.2GHz processor for about $300 on eBay. I know you said White MacBook but this is probably the closest thing you will get to with $300.
David