I haven't followed 356 values that closely . . . . but I do remember looking for my 356 20 years ago. I suspect values are tied to the supply of cars, as much as anything.
When you consider how few 356s were built to begin with (versus the 911), and all the mis-adventures that these cars could get into over the intervening decades (before they were considered anything more than Hi-Po VWs), it amazes me how many of these cars survived at all, in any condition, even back then. 356s have a completely different level of assembly and finishing, from most cars, including 911s. 356s with 'original' bodywork are super-scarce ---these are not cars I would look forward to restoring. Believe me when I say that a 'good' 356 is a very rare car.
As for Speedsters, D Convertibles, and Roadsters ---- these were never built in large numbers to begin with. Factor in attrition, not to mention all the 'restorations' and 're-dos' that have taken place since . . . . then the values for good cars would come as no big surprise. The number of vehicles available --- let alone being offerred for sale --- could not be a big one.
As for Targa values, well . . . Targas remind me of Porsche's 356 Cabriolet. I guess an open car is always fun to have. But, to me, on-the-road performance is what makes a Porsche. To me, Targas, like Cabs, offer a less-than-elegant profile to go with an 'open air' experience that I don't think is worth the performance cost. 356 Speedsters and Roadsters are lower-content cars by design, but offer some performance for their lower-spec, whereas the Cabs are deluxe, extra-everything cars that are heavier to boot. Cabs are top-of-the-line, and that's what they're built for. What's a Targa built for? On-the-road perrformance? Or something else?
I suspect that, when we're talking about longhood values, given the number of cars available, and their relative affordability/accessibility, there'll always be more butts than seats --- for coupes or Targas --- so values will always stay high, either way.
For what it's worth, I remember guys talking about buying and socking away '67 S soft-top Targas, like, 20 years ago. 'Be worth a ton of money, some day.'
Didn't buy one then.
Wouldn't buy one now.
Rick Kreiskott