I have it on good authority that Jacobs Well is a good park. It's a nice quiet out-of-the-way place on the bay. I believe the price isn't too bad either.
James Sharp Park near Canungra is a good spot. On the banks of the Coomera River. It was $6 / person / night about 6 months ago. Tiolets, no showers, suitable for big rigs. It's busy on weekends so turn up through the week to get a good spot.