Well, if you want to see a budget system, mine cost about $300 for the whole lot:
$100 2nd hand Pioneer MP3 head unit
$35 2nd hand Panasonic 6.5" 2-ways in the back parcel shelf (don't buy these, they've got no bass)
$0 2nd hand Pioneer 6.5" twin-cone speakers in the front doors (from my corsair wreck)
$6 pair of mylar tweeters from Jaycar (sound as good as the $30 sets in Strathfield, but without the case)
$73 Solid Audio ~100W 2ch amp from JB HiFi
$25 10" Power Acoustik sub from JB
$52 8ga wiring kit
$291 for the lot, which sounds way better IMO than the stock sound system in a mate's BA Falcon.
And I'm very happy with the sound I get out of it. I need another amp though for the fronts, 4ohm tweeters and 4ohm full-ranges running in parallel doesn't make for a happy head unit
Shop around for your stuff, I find JB HiFi have some great specials on from time to time, and an Autobarn near me has had some crazy midnight sales I've heard of the day after they happened
And, surprisingly, I have no trouble with engine noise from the amp after grounding the RCAs at the head unit. Apparently Pioneer head units have dodgy grounds on the RCAs in their head units.