Liquid,
I have just installed some gear into my r33 and while it would not suit the sound quality conscious it is nice, clear, loud enough for me and most importantly for some of us, inexpensive.
I purchased a JVC 995 MP3 headunit new off e-bay for $400
(cheapest h/u i could find at the time that was MP3 compat and had 4v pre-outs and a controllable sub out)
For the doors i purchased a set of Kenwood 16cm 2-ways $115
Rear (not necessary but i thought, what the heck) i have Pioneer 16cm 2-ways approx $100
Sub, Clarion 12" dual voice coil package deal in enclosure from Autobarn $249
I am driving the sub off an old but faithful Alpine amp that i have had for some time so saved a bit of money there.
So thats a total of $865 if you were to find a nice reasonably priced amp (jaycar maybe) for the sub you wouldn't be too far over your budget. I am sure if you shopped hard you could find all the above cheaper.
As i said above, not perfect sound, not competition quality but makes plenty of noise and not distorted.
Oh yeah, the above speakers all went straight into the factory mounts.
Hope this helps
Gary