Because I am borderline psycho and because I like you, hamiltonau, I have made the following graphical representations of Carsales.com.au data.
You can quite easily see the pricing trends with the three models of car, it's a shame there wasn't more samples for the R33's as the graphs look a little patchy, but you can still see a nice trend. It will be one of my biggest regrets that I didn't have time to insert a nice bell-shaped-curve on the first graph. Would have fit really well, perfect data grouping, anywhoo, you can see that the redbook valuation is a bit bullshit. Remember, in this market you can always budget on knocking a further $1500-$2000 from the asking price.