Hock 'em on Craigslist, and buy something vintage. My father managed a pair of the same mid-sized KLHs his father still uses for $40 on eBay, and I sold a later pair of Bose cubes I bought at a garage sale for $4 for $80. I once managed a pair of Mordaunt-Short 5.20s for $1.50, but that was through a combination of good luck and obsessive searching.

Remember, while new speakers are better than old speakers in almost every way, you can get some of the better vintage stuff for boom-box money.