Generally, stamp duty is calculated on a sliding scale based on the value of the property: the higher the property value, the higher the percentage of duty payable. Some states apply a flat rate after a certain property value threshold is reached.
For example, the rate might be 1.25% on the portion of the property value up to $250,000, then 2.5% on the portion from $250,001 to $500,000, and so on.
This progressive nature of stamp duty calculation means that more expensive properties incur a higher rate of duty.