It's no coffee cup speaker, but PET bottles do, in fact, make a pretty decent shell for a couple of small speakers. Plus, the standardised shape means they will fit in a lot of existing holders.
Yeah, $US15 speakers from CVS are not going to crank out a lot of sound, but it is definitely easy on the budget—and it doesn't require a ton of time and skill to accomplish. Hit instructables for details on how to make one yourself. [instructables via Make]