Well, as with all things it down to supply and demand..
With the Aussie Muscle Car it seems there is more supply than Demand at the moment so prices will drop..
There is at least 7-8 GTHO Phase 3s on the Market at the moment ranging from Approx 700K to 1 Million, which tells you there is a over supply of these Cars..

I feel the bubble as burst, especially at the top end of the Muscle Car Market
Some of the lesser Muscle Cars under 100K don't seem to be as bad off, especially if it is a Mint Low Mileage example..
An Example of this was the HT GTS 186s that sold at the Shannons Auction in Melbourne earlier this month for 75K, which by all accounts was a excellent Monaro with low miles (approx 35,000)
It will be interesting to see what happens at The Sydney Motor Show Auction in Oct, as there will be a few varied Aussie Muscle Cars up for Auction. Already there is two GTHOs. a VC Brock, XA GT RPO83 and more not listed yet.