My Two Cents
I agree with Travis. If you look on Cars.com, '08 Granturismos are already hitting $69,900 asking price. Who knows how low you can go by negotiating.
If the '08s keep depreciating like they already have been, you would expect to see a $10,000 per year drop for the '08s from here on, with a bottoming out somewhere, probably in the $45,000 range.
I see the '09s following suit, but bottoming out a little higher, probably in the mid $50,000 range, given the 4.7 liter engine, and other options you can get in the S with the F1 and S with the automatic.
I think both developments are sad for the marque, but that's where I see things going.
The one potential saving grace is that it is possible these Masers, unlike certain models of BMWs, Mercedes, Audis, etc. have a bottom under which they will not fall. For example, my 2007 SL 55 AMG does not appear to have a bottom, based on asking pricess of earlier-year SL AMG models I'm tracking on Cars.com. Given that, I will probably keep the SL 55 AMG for as long as it is not costly to maintain, in order to get as much value out of it as I can.
But if you buy a used Granturismo in the $70,000 or $80,000 range, and it hits a floor of $40,000 or so, that's really not that bad, compared to what might happen if one buys a comparable German car.
Just my two cents, anyway.