I have been looking for GPS and this seems to be the cheapest
The cheapest I found online is ryda : http://www.ryda.com.au/TomTom-GO730-GPS-Navigation-Sat-Nav-p…
For : $478.95 then you need to add Shipping: $16.65 Aust Wide (Insured)
They have another set that comes with value pack : http://www.ryda.com.au/TomTom-GO730-GPS-Navigation-Value-Pac… for $499.95 + shipping $16.95
So depends whether you need the value pack or not. I think Myer price is pretty good.