I've seen this come up quite a bit lately, and even though I'm in the "never selling my Z" camp, I thought I'd do a little research to see how the Z's value has held up when compared to a few other cars (even though I'm a little late to the party on this thread). Bear with me on this, as there aren't many natural competitors to the 370Z, so I threw in a few sports coupes along with a couple BMWs and some Japanese entry-level lux models to see how these cars' values all stacked up 5 years on.
A few caveats: all MSRPs and KBB values were for the 'base' trims of each model with standard equipment. I used the average KBB trade-in value and the maximum KBB private party retail value to compute residual values. Residual in this case is just the amount of original MSRP retained (trade/MSRP and retail/MSRP).
Interestingly, the Z scores just above average with the
fourth-highest trade-in residual (from this admittedly small sampling). The three worst residuals from KBB average trade-in value, in order, were the S2000 (likely because it was discontinued?), 335i and Mustang GT. On the other hand the M3, IS350 and Corvette serve to bring the average up with strong residuals.
Aside from a pronounced recent drop as described by the OP, it appears that there's some sticker shock in seeing the value of a 5-year old car, one that started at a MSRP just under $30k. Otherwise, aside from some anecdotes about offers and trade scenarios, it appears that the Z's value is holding up nicely (at least from a theoretical appraisal book value standpoint). Bake in variables like dealers who don't move many Zs to seasonal sales concerns in colder regions, and you can see how the value could take a hit.