You will need:
-a subwoofer (one 12" is good)
-an amplifier
-a box (general rule: sealed sounds the best, but bandpass and ported can put out more bass)
-a bunch of wiring
The easiest setup is this:
Mount the amp to the back of your rear seats. Then run a decent size ground (negative) cable (easiest place to attach is to the same bolt that the seatbelt uses). The positive has to be run to the battery. You can run this under the door molding along the side to get to the front of the car. You then need to get through the firewall which sometimes involves taking the dash off (not really a big deal). Make sure you put a fuse on the positive power wire very near to the battery.
To feed a signal to the amp, you can tap into the rear speakers. This is not the best way to do it, but if you aren't going to replace the deck, its the only way. You have to get an amp that accepts high-level inputs.
Then just put the sub in the box and attach it to the amp, set the levels, and there ya go!
You should be able to do that install in a few hours even if you don't know what you're doing
As for what to buy, $200 is a little low. Right now, I have an entry level sub and amp. Phoenix Gold
QX12 in a sealed box, and a
QX2180 amp. I'm thinking of selling this, and would probably get about $350cdn ~ $230US. If you want to buy used gear, you can get something decent for around that price.
EDIT: links