I have a game ball used in the 1975 AFC divisional playoff game played between the Steelers and Colts. The ball is signed by most of the roster including six hall of famers: Terry Bradshaw, Jack Lambert, Joe Greene, John Stallworth, Jack Ham and Mel Blount. The signatures have started to fade but it is still in pretty good shape. I also know this was the game the Terrible Towel was invented. How much would a ball like this be worth?