The autograph market is over run with forgeries and fakes. There are web sites which most people think run a lot of fake autographs. Coaches Corner is a good example. One of their principal authenticators, a guy named Morales has been banned by ebay. I was recently in Vegas and most of the autographs I saw on the strip were fakes.
Most serious collectors want an autograph that has been certified as authentic by one of the better companies. PSA and SGC are examples of companies most people trust (but they do make mistakes).
The signature without a letter of authenticity from one of the respected companies will probably bring about $100 to $200. From the scan it looks like a black and white photo which does detract a little from its value. Size also matters, an 8 by 10 or 11 by 14 will have more value than a 5 by 7. Most people want something they can matte and frame and will look good on their wall. With the certification it may double in value. (the cost of having it certified is not trivial). Certification will also up your prospective customer base. Without certification you have either those collectors who are very knowledgable and can determine for themselves if it is authentic and the clueless who just want something on their wall. I don't think ebay will take an autograph any more without a valid letter of authenticity.
Thanks Tachyon. I was just curious as to it's value (he doesn't want to sell it). Walter signed it for him in his presence so it's real. The picture is a B/W photo from the early '80's I believe and it's a 8x10.
Best player to ever play the game, in my era. His yardage record was broken by a guy that ran behind an all pro line most of his career. Walter Payton did most of it on his own, and did not run out of bounds like Emmitt Smith did. Smith stayed in football just to break the record. To bad Barry Sanders retired early.