lol, a lot, i would say a brief overview:

headunit 200-300
front speakers-100
rear speakers-100
amp-anywhere from 200-500
sub-100-300
box-around100
amp install kit-2gauge, prob 100
installation-100

if you cant install anything yourself you can use crutchfield's install cards, they are nice, but in the mustang the headunit is easy enough, and so are the speakers, the only thing thats ever hard is an amp, cause you gotta run the rca's from your headunit to the amp, and the ground/power cables on the amp. i would say www.crutchfield.com has the best deals for about everything because they include all your installation parts for free. those are just a rough estimate, but it can get pricey if you want all jl audio or something