How much is it normally cost to install headunit + speakers + subwoofer?
You'd also most likely need an amplifier if you are going down the subwoofer route. If you're buying an aftermarket head unit, it'll probably have some (small amount of) amplification which will be sufficient for speakers.
I personally went for the 4 channel amplifier with 2 of the channels powering front speakers + tweeters (as I kept the original head unit) and the 2 other channels bridging to power the subwoofer - but this isn't for everyone and is a bit of a compromise.
In your case though, I'd just change the head unit (and ensure it has RCA pre-outs - this is essential to enable you to upgrade later on) and if you really want the bass, then get a monoblock amplifier (i.e. just for subwoofer) and a sub. I haven't looked into this in a while but for a budget (but not crap) set up, I'd probably expect to pay around $150-250 for the head unit depending on features/look/etc. Speakers of even $50 would be an improvement over what is included from factory in your car but again depending on head unit (or if you plan to get amplifiers) you could go for more expensive speakers or components/splits. Amplifier will be at least $150 if on sale and a subwoofer with case another $150?
Not sure if Strathfield Car Radios is still around but them and JB Hifi would be where I'd look. And Autobarn.