Some nice prices were recently realized in a recent auction or two over at Heritage for a couple of prototypes that were created as the 1968 Topps 3-D Baseball set was being developed and, while I don't typically focus on values here (and also discussed some of these about three months ago), it seems instructive to do so here.
This example of the Brooks Robinson prototype, which has a tear at the bottom, is now in a PSA Authentic slab and went for a cool 60 Grand:
I believe that is a record for this card, where a mere handful exist, with a definite upward trendline on pricing. Also of note, was $21 K for this nom-de plumed Yaz in the same auction:
Meanwhile, the Roberto Clemente prototype I discussed here only last month, went for a paltry $3,145.20:
Granted, it was on illustration board and a mock-up vs. a mostly finished prototype, I think it went w-a-a-a-y cheap, especially considering Clemente's staying power in the hobby marketplace.
The 1968 3-D's clearly remain popular, whether they are are proofs, prototypes or actual issued cards. Whether or not the more esoteric mockups and the like from other sets will catch up remains to be seen but as prices move up for the 3-D's production materials, I suspect that will raise the price of other such items as well, at least for Hall-of-Famers.
Happy Festivus!