I've got to ask something, why are second hand cars so expensive in Australia?!!
Did you only go to the car yards? If so, don't
. They're populated by one of Australia's most venomous animals; the used car salesman. Brown snakes have nothing on those guys.
Get your son to have a look in the Courier Mail classifieds or on carsales.com.au and you should find they're cheaper, though not as cheap as in the UK. Apologies if you've already done that and are still swooning at the cost.
Re flying; did you look at any of the Coolangatta schools? There are a couple there; other people here will know more about them than I do.
Have a great holiday. I haven't seen it but the Outback Spectacular is meant to be a lot of fun (albeit touristy) It's over the road from Dreamworld and not nearly as pricey ($100 per head with steak dinner included).
Gold Coast Show, Theatre Restaurant, Family Attractions - Australian Outback Spectacular