As best I can figure, extracting the secret from a Mondex card gives you not merely the money from the card, but the "digital plates" with which to mint arbitrarily much more money. I only say this because the only protocol the I can think of that fits what we do know of Mondex has this problem. This fault does not plague Chaum cash.
There is no way to know for sure, since Mondex won't release the specs, but just about everybody I talk with that knows at least something about their system agrees that if you crack a Mondex card, you will likely be able to mint money. Since Mondex allows transfers from card to card to card, it may be a long time indeed before the breech is disovered. If I were a Mondex issuer, I would be worried. Very worried. --Lucky