You need
@Red Leg to weigh in on this thread, he is truly, truly the expert on S2s.
However, I do know that the large bores with the modest engraving were being liquidated to dealers for $5000-$6500 or less and being rapidly flipped on "blowout" for $6000-$8000 awhile back. The really nicely engraved boxlock-no-sideplates models were selling under $10k.
Zero idea what a prolifically engraved sideplated model like the picture might be worth, but I've found that prolifically engraved double rifles with every extra included and exhibition wood don't sell for much more than the ugly knockabout version with border engraving of the same model...as price goes up, number of potential buyers diminish greatly. (thus, $15k MSRP model might sell for $7k, $50k MSRP model might sell for $13k)
If you love S2s, the price doesn't matter. If you don't love S2s...you only start to care when the price is so good it overcomes your preferences for something with different aesthetics. (and I've gotten enough good deals on guns that I've shot some very ugly guns just because I got a $10k discount on them, including my soulless turd of a Spanish shotgun that I tote everywhere.) The dealers that have the S2s left as-new bought them for a song and are trying to double or triple their money on them. The true deals came and went with the dealers that flipped them for a grand profit in a month's time dried up.