Proof Set Error Packaging

A friend has asked me if she had an error coin. She has a 1972 proof set with what she thought was a two reverse sided dime. I told her that could not happen from the mint and suggested she has a proof set with two dimes face to face. She looked more closely and that is exactly what had happened. I personally have not seen such an error in packaging. She was asking if that increased the value of her set. I told her that it would probably carry a premium to the right collector. Does anyone have a idea of the extra value this might carry? Sorry, I do not have pictures at this time.