The market value of an athlete's performance is only higher because it is considered more valuable. When prize money value is increased, the result is a higher value on athletic performance, prompting better sponsorships, offers and even fan attention. Equal prize money also demonstrates the progressive values that athletes need to emulate as well.
Yes, it is fair to pay market value for an athlete, regardless of gender. Professional sport is not a charity. Organizations and athletes participate in professional sports because they can make money doing so. In some cases, women are more highly paid in a given sport. It is not fair to punish them any more than it is fair to punish a more highly paid man.