Good s/h prices I put down to:
1) Rarity, in comparison to 350's. Yes, you can still buy new 370s but the price + tax + petrol consumption compared to other new cars offering similar 'bar stool' performance is never going to help sales now.
2) One of the last (non supercar) V6 engined sports cars with grunt and....now s/h ones are more affordable with new ones not being around for much longer I suspect with Nissan having deleted the roadster from the line-up.
3) Tuning potential that has only come to peeps attention in the last couple of years.
4) Has overcome the look put-downs, when compared to the 350's in its early years, now that the modding potential has shown how good they can look.
I would have another in a shot if our circumstances permitted!
So good news for current owners and for those getting one, IMO.