The National Association of Realtors in Australia has made some regulations on what an affordable house should includes Mortgage rates, Housing prices and Payment of mortgage according to personal income percentage. With these three factors in mind, buyers can possess a house.  
In conclusion, In case you have no idea of what amount is considered good enough to purchase an affordable house, you can forward your queries to the Accrue real estate agency. They deal in helping people find budget-friendly houses in good locations with standard infrastructures.