when you do find a place, make sure they evacuate the aircon system for a good 30-40 minutes before regassing (theyll know what youre talking about if you say that to them). this will not only get rid of the old gas, but will also get rid of any moisture in the system.
in syd an evacuation and regas is around $110 or so..
too many dodgy places just try to do a gas top up only which is why you get cheap prices like $89, and it wont work the best .... but you get what you pay for