It really depends on where you take your car to.
Ive heard Autobarn charges roughly $80 to install a headunit.
Im an auto electrician and my workshop charges $90,thats our hourly rate..but i agree if you want a cheap way out much better to get a friend to do it, or find a wiring diagram to do it...
Go to an auto retail store and purchase a circuit test light.
Clamp the clip to the key or ground of the car and probe the terminals or wiring on the car with ignition off...if the test light lights up thats your battery + (yellow on aftermarket units).
do the same with the key on acc, when the test light lights un on a different terminal or wire that your acc wire (red wire on unit)
the ground wire is exciting to find as some peope get tricked by the original head unit illumination circuit. Now clip the test light to battery positive feed and probe the wires until the light glows, now turn the parklights on, if the test light remains glowing thats your ground wire. if the test light goes out then thats your illumination feed, keep probing untill you find the ground...
may sound tricky but seriously its a piece of p***