Wow.


Anyway, with car audio, everything is relative. What is inexpensive to me may be expensive to you. What sounds good to me may sound like crap to you. So remember that.

Here is what I would do:

-Upgrade the headunit. You can pick up a nice one with plenty of features for $200-$250.
-Upgrade the front speakers with a decent set of comps for $150.
-If you want you can keep the rear speakers as is, or upgrade them to an inexpensive pair for ~$75-100. I choose to leave nothing in the rear. Personal perference.
-Then you will need something for the low frequencies. I have a 10" sub in my Integra sedan, and it is enough bass for me. Check out Elemental Designs. You can get a 10" for under $100 now through their group buy deal.
-Then to make the best out of that sub you need a good box for it. $100 can get you a custom built box, or you can build your own for about $30 if you have all the tools already.
-Then comes the amp. I personally would get a 4 ch amp since I would be on a budget. First 2 channels for the fronts, bridge the last 2 for the sub. The rear fill can be powered from the deck.
-Then you will need wiring.