Step 1:
Book a Free Dental Health Check for Pet Dogs & Cats @ Greencross Vets
Step 2:
Receive $20 OFF Royal Canin Dental Range in-clinic
Example:
1.5kg Royal Canin Veterinary Diet Dental Small Breed Dog Food $21 after discount
Not sure if all-time cheapest, but compared to what's available now, it's at least $10 cheaper than other shops.
My cats think the Royal Canin Oral dental food is a treat. My Vet recommended it rather than Greenies. They get this as well as the Feline natural wet food and the Ziwipeak dry food.