You could always recycle...

cardboard body kit. free
Spaghetti tin can welded onto the end of your exhaust. free
Go to the bush and rub your rims so much with dirt they turn black. free
Tape your bonnet with black and dark grey electrical tape so it looks like carbon fibre
Get some more electrical tape and do some racing stripes down the side of your car

Cost - $10 for electrical tape