These are approx costs, that Autobarn,strathfield etc charge:
headunit - $66-80
Speakers - $40- more for splits maybe $50-60 if they have ext crossovers.
Amp - $70
And if u dont have an amp wiring kit that will be an extra cost.
Honestly though just go get some quotes, drive there and show them your car if you like. They're pretty much flat prices for everything,except if they can see there being complications.