Assuming you can't fit component speakers...

$130 Head unit

$55 Speakers

$180 Amp

$115 Subwoofer

$40 Box

$60 Amp kit

$8 More rca cable

Order your head unit by phone to make sure you get any controls and harnesses you may need. Forget the rear speakers, bridge channels 3&4 into the subwoofer. Don't forget to seal and deaden doors