What's monaro worth?

My dad is the owner of a '72 Holden Monaro. It is a one-owner vehicle with the original factory-fitted Chev 350 and about 130,000km on the clock. The only modifications are Corvette exhaust headers and the wheels. I would like to investigate selling it, but Shannons told me about six months ago that it would fetch about $12,000-$15,000 at auction. This seemed too low since Dad knocked back $17,000 about 10 years ago. What are your thoughts on its value?