You really have two cost components.

One is what it takes to get the OEM source units (Nav, CD/Tape, Bluetooth, AM/FM, Sirius, etc.) up to par with an aftermarket head unit as far as adding amps and speakers is concerned.

The other is what amps and speakers you get, and how you have them installed.

I'd say that you are looking at about $400 in parts to get to a full-range pre-amp signal, and then from $250 to $800 to add that optional EQ to get that signal as flat as possible from a frequency-response point of view.

For installation on that, I'd look at around $500 for the install and tuning, especially if you have a front-mounted fader to install (please note that earlier years may very well have rear signals without the "holes" in the response, which would eliminate the outboard fader but probably add another $150 in parts cost for those additional channels.

All that is just to get you to the point that you can get excellent-sounding speakers and amps and have them getting a good signal that they can use to sound the way they should. Obviously, if you are looking at doing an upgrade with entry-level gear, it's probably not going to pencil out.