A souvenir postcard of the 1929 'Bunion Derby' featuring the winner John Salo. This was a race from New York to Los Angeles, a distance of 3,665 miles. Always a popular collectible, runners have been signing for years and dealers usually have them as a supplement to their main sports offerings. You must have trust in the dealer that an autograph is genuine. I don't think the limited running market is subject to the forgeries found in baseball, but you can't be sure of authenticity unless the dealer provides proof.
Prices seem arbitrary, depending on the knowledge of the dealer.
It has been known for a few years in the running community that Mamo is being unjustly held in prison. During the Atlanta Olympics Mamo Wolde's tragic story was broadcast. Autographs come in different forms with different values. A handwritten, signed letter with interesting content would be prized. A plain card with just a signature is standard.