Here is some more info on these, I imagine there are some who are interested so I'll be the "gopher" today.
PSA pop report shows 78 graded total.
58 of them are PSA 3s or lower.
54 of them appear in the "detailed" pop report, meaning it specifies the back series.
Only 2 are Piedmonts according to the "detailed" pop (both of those are "A"s, so it's not tough to infer that there can't be too many more Piedmont's out there).
Card on the left is the first Goldin Plank, a Sweet Cap 350 Series back, which is the most common.
This card sold for $162,360 including the buyer's premium.
Compare that to the card on the right, which is from Heritage in 2019, which is a 150 Series back. This back is quite a bit more rare and is actually the highest graded of these in the "detail" pop report which some might argue warrants a premium.
It sold for $252,000 in 2019. I guarantee this card would sell for quite a bit more today.
Card on the left is the other Goldin Plank. It is a 150 Series back. The "Authentic" grade makes sense, and I agree with the above assessment this card was likely printers scrap in actuality.
This one sold for $34,440 including buyer's premium.
Compare it to the one on the right, which is from REA in April of this year. It is a Series 350 back (so more common), also graded "Authentic" but also "Altered" due to the bottom right edge (more on that in a moment).
This card sold for $33,600 in April (I own it).
I am far from an expert on this card, however here are some reasons I think these prices are not actually suspect at all. First, there are only 78 of these in slabs - compared to 36 Wagners which most would estimate are all $1,000,000 or more in value at this point, so Plank becomes an "affordable" second choice. Also, there are easily at least 78 collectors out there either (1) working on the "monster" set, or (2) working on a HOF portrait subset, or (3) Plank player collectors - all of these people would want this card, in addition to all the other people who would just buy it because of its rarity.
This is definitely one of the hobby "grail" cards, and when taken in perspective to the price of some of the modern cards, it comparatively is quite "cheap" on the particularly considering the overall supply. When you then consider "eye appeal", population gets even smaller.
Anyway, that's my two cents...