stamp duty is based on a % of the market value. I just purchased my 350gt last week and market value was $26,000 and my stamp duty was $750.
Lucky me i changed the value of the vehicle on the bit of paper u take into department of transport to $12,000 which halved my stamp duty fee. Just be careful u don't put the value of the car to something ridiculous like $2000 or else you WILL get tracked down by the "stamp duty people" idk who they are and you'll be pretty much screwed, i guess it can be called fraud.
goodluck!