The cost of living is determined to be what it costs to maintain a certain standard of living.
There are complicated formulas and indices used to calculate the cost of living in a city which result in a number which, on the whole, won’t mean much to you and will change depending on which cities you are comparing. So if I told you the cost of living in Adelaide is 98, you’d still be as much in the dark as before. What it boils down to is this: stuff is cheaper in Adelaide.
To buy 2 litres of milk in Sydney you can expect to pay $3.06, while in Adelaide it is $2.42. It’s not just groceries that are cheaper either. Average petrol prices in Adelaide are also lower. In Sydney fuel averages 137.6 cents per litre, in Melbourne 134.4 but in Adelaide it’s only 132.5. If you prefer to take public transport then Adelaide turns out to be cheaper here as well. For a 10-trip adult bus or train ticket it is $30.90. In Sydney the same thing costs $36.00. For an adult ticket to the cinema in Melbourne you can expect to pay $17.50. In Adelaide it’s $16.50.
All these differences in price add up, making Adelaide’s cost of living one the lowest in the country.