The cost of registration in Western Austrlia is calculated on the weight of your vehicle. To put it simply, the more your car weighs, the more you're going to pay.
The most expensive i've seen rego go for is $17,000 for 12months for a mobile crane
The weight of your vehicle on our records can vary because when each vehicle is weighed it can differ due to but not limited to stereo equiptment, aftermarket parts (some wheels may be heavier etc etc), how much fuel is in your tank at the time and so on.
However in saying that, the difference in the cost of registration between a car that weighs 1300kg to a car that weighs 1500kg is not that much.
Of course there are other factors such as coupe/sedan/4wd/etc, 4/6/8/10/12cylinder, as that all affects the insurance premium and insurance class the govt charges for compulsory 3rd party.
That's the answer to your question, in a nutshell of course
I should no, I work for the department