The OP is that guy who said he bought the Alero from his girlfriend's dad for 80 bucks.
Even if you buy some cheap ass 15s you'll still need to make/buy a box, power/speaker wires, an amp and a headunit. Go on your local craigslist and look for used stuff if you're that strapped for cash.
My Crystals bump! 350rms @ 4 ohm SVC $50 each +S&H pretty rare subs as Crystal Mobile Sound is no longer in bizzz :/ The Comp series was and still is sought after... These are the CSe 15s...