heres the cheapest setup you could possibly have, which i have
by chance, find a 12 inch sub (jensen xs12) at a garage sale for $20, talk to your cousin who has some 3/4 inch plywood laying around and a bunch of screws and make your own box. go to a pawn shop and pick up an amp for $40 (pioneer 240 watts max output, nothing special, just enough to get the job done). go to home depot and buy some 14 gauge speaker wire (i got 40 ft for 1/2 price @ $0.14/ft), and some 12 gauge extension cable wire (8 ft @ $0.61/ft), this cost me $11. get lucky and have the rest of the wires laying around in some boxes (i only needed 2 rather long rca cables and some thin wire to act as the signal wire). some spraypaint for the box is another $4. get an aftermarket head unit (naxa) for christmas, and install everything in your garage
total investment=$75 and start to bumpin time about 6 hours. this includes mearsuing and making the box, drive time to different stores and the wiring. i still have to paint the box, and hide the wires
the amp is underneath the passanger seat and the sub is in the trunk
if you have any questions about the install, let me know and i will be glad to help