what kind of sound should i expect from an zorst on the gs500, how much (AUD) is it worth it?

The GS sounds like a pissed off lawnmower no matter what exhaust you put on it.  

Well, I've heard some aftermarket exhausts (the WRX can comes to mind haha) that will make it sound like a dirtbike.   :lol:

GS sounds very much like a two cylinder bike...wait....one....two...oh my.... :lol:

http://www.formula1mc.com.au/

FULL SYSTEM WITH ALLOY CAN   $650 AUSSIE DOLLARS
FULL SYSTEM WITH CARBON FIBRE CAN  $725 AUSSIE DOLLARS

SOUNDS AWESOME, PERFORMS WELL.

Like others have said a parallel twin is probably not going to sound very mean no matter what system you put on there, aftermarket exhaust systems do sound much better than the stock but personally I'm just waiting until I get myself a V-Twin before I invest in any sound enhancing peices.