The price of a Bullmastiff can be all over the place, depending on how old they are, their pedigree, and where they're from. You're looking at shelling out between $1000 to $4000 AUD for a top-notch pup from a breeder who really knows their stuff.
List of Bullmastiff breeders in Australia
Canberra
BLAQVEIL, Bullmastiff, Canberra ACT, 0434365569
New South Walles
GRIZZMUFFLEY, Bullmastiff, Bungendore near Canberra NSW, 0420 938 026
BULLARMOUR, Bullmastiff, Waterview Heights 2460 NSW, 0407042388 or 0429447427, bullarmour@bigpond.com