the stereo its self can be pretty cheap 70 - 130 bucks but I don't know if its the stereo or the speakers.

u could get a complete set that starts from around 80$ to like 500$ it should include installation too. Depends on the Brand u want ( I recommend getting Boss Audio)

Well you first have to figure out what size speakers are in your door to begin with. and if you want something above average go with crescendo audio's midbass and supertweets, then you have to figure out if you want to power them with an amp usually its a 4-channel amp and again I would go with crescendo audio for that as well, then there is your head unit, you are going to want to upgrade that to one that is more powerful than your stock headunit and can take multiple amps but in your case just one I would go with pioneer or alpine for that, then you have wire for your amp and speakers, you will probably need 4 gauge power and ground and then 14 gauge speaker wire and remote wire

you can get a good head unit for average of 80$
Speakers depending on size 50-100$ for decent ones not going to be competition quality but still good. if you want a sub or not changes things to. depends on your overall preference of bass or sound quality. id say less then 500$

seckkb1 wrote u could get a complete set that starts from around 80$ to like 500$ it should include installation too. Depends on the Brand u want ( I recommend getting Boss Audio)

I would not recommend boss audio!! Horrible reviews and crap products

Do you only want a head unit and speakers or do you also want a sub?

just get a sub and amp...a sub makes a biiiig difference.
then if you can afford it just get two rear speakers and your golden.
No point in replacing all of your speakers

I have to agree get a sub and amp, and join it up with speakers already in the car.
Pioneer have some brillant products, and to be fair for this your looking 500 +

Some of the best subs you can get are from MTX, DAD, Alpine or Kenwood. The amp I would recommend for 2 12's would be a kicker amp. Their subs aren't the hardest hitting but they have great amps.