Wow they screwed you over big time!
Did you not agree on the trade price prior to signing the contract? I'm guessing no? If a price wasn't agreed upon then I don't think you can form a binding contract - it just gives the dealer too much power to charge what ever he wants. Go over your contract and make sure it doesn't mention the price.
This might be of some help:
http://www.fairtrading.qld.gov.au/motor-industry.htm