Non commercial only:

case(int(({DutyValue}+99.99)/100),x>30,x*4-60,x>20,x*3-30,x>10,x*2-10,x>5,x,5)

https://www.revenuesa.sa.gov.au/stamp-duty-vehicles/rates

Restatement of formula from website

• Minimum $5
• duty value <=1000 : $1 for every $100 or part thereof
• Duty value <=2000 : $10 + $2 for every $100 or part thereof over 1000
• Duty value <=3000 : $30 + $3 for every $100 or part thereof over 2000
• Otherwise : $60 + $4 for every $100 or part thereof over 3000

Restatement / Refactor to simplify the math

• Round duty up to nearest $100
• duty value <=500 : $5
• duty value <=1000 : $1 for every $100
• Duty value <=2000 : $2 for every $100 - $10
• Duty value <=3000 : $3 for every $100 - $30
• Otherwise : $4 for every $100 - $60