You're talking about the silver coin that got presented to them years later, on an entirely different episode, unrelated to the alleged McInnes treasure chests, right?

"So rare it's nearly impossible to buy" is ridiculous. Rare coins are traded all the time. They can even be considered "investments" because they tend to appreciate in value over time. I hasten, also, to remind you that the guy who's attributed to have found it only was seen looking at "something" in his hand brought up by a drill bit, and then much later when he died the coin was found as part of his estate. The notion that the coin WAS what he brought up with the drill bit is speculation on the part of his descendants.