What is a 12 GA superposed with a mild rust due to salt wood stock worth?

These "salt wood stock" guns are, depending on the extent of the damage, worth considerably less than an equal condition Superposed without the salt problem. There are a lot of variations of the Superposed that can also affect its value. My advice would be to consult one of the firearm pricing guides avalible at any large book store, keeping in mind that these pricing guides generally quote pricing that you might expcept to pay for the firearm at a gunshop not what they will give you for that gun.