Depends a LOT on condition, if the neck is straight, do they have any wood cracks? Are they all original? Original cases? If you have a store that sells Martins in your area, chances are that they will have a salesperson that can value them for you. I have seen D-28's from $1500 - $3500 and D-35's from $1500 - $4500
depending on condition for Martins made in those years. Are you looking to sell ??