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. These are usually less valuable. Emil Zatopek, triple Gold Medalist in the 1952 Helsinki Games. Zola Budd, famous for her collision with Mary Decker Slaney.