It's actually a simple equation:
Instalment - $500-800, depending on your downpayment, no. of years
loan, etc, but i believe most car buyers now pay the least and
spread the loan over, say 7 years. This is likely to result in
about $600-700 for a normal Jap or Korean car..
Petrol - depends on usage, but typically $100-400
Road tax - about $50-100 a month
Insurance - about $1500-2500 a year
Parking - fixed HDB about $65-90, variable for your other
trips
Just take an average of each of the above components, and you are
likely to end up with about $1000/month.
