There is an 1971 s no s proof nickel in a proof set and only Like 1647 are known. I have two 2000 us mint set that are missing one of the state quarters one is a Denver set the other a Philadelphia . There are no premium paid for such sets just some people like collecting them as an error. You have a Denver mint set with a Philadelphia nickel. Yes different and yes an error a human error at the mint. Not a coin that was minted in Denver that did not have a mint mark D on it. Sorry neat find but only as a conversation piece .
So it doesn't add a "bit of value" other than a newbie learning experience that its a packaging screw up and they shouldn't have paid $25 for it but could have had it for $3.00?
He paid $25.00 for it? Yikes! No, that's an expensive learning experience. I didn't mean to actually go out and buy one, just keep it if you already had it at the $3.00 price point.
I have no idea what was paid for the set but I do know that this situation occurs due to how the sets were assembled and adds no real numismatic value to the set.