With all the controversy surrounding Alex Rodriguez surpassing Willie Mays for fourth-place on the career home run list with #661 on May 14, it is interesting to note that at least as of 2001, the whereabouts of the ball that Mays hit for his 500th career home run in 1965 are unknown.
Mays' milestone came on Sept. 13 at the newly opened Astrodome. Leading off the 4th inning he homered to center field off Don Nottebart to tie the score 1-1. The distance was estimated at 410 feet. The Giants went on to win the game 5-1, running their winning streak to 11 straight on the way to 14.
The ball was grabbed up by Mrs. Sallie B. Norman, a local woman. Eddie Logan, the Giants' equipment manager, rushed to her seat in left-center and offered her an autographed ball in trade, but she held onto the home run ball.
After the game, Mays told the media, "Don't make her look bad. Don't make her do nothing. It's hers and she can do what she wants." Mays was given the woman's phone number but he said he doubted he would call her. "She can have it," he said.
On April 20, 2001, a few days after Barry Bonds' 500th homer, Mays told USA Today, "I don't know where mine is. I wish I could find it."
The Bonds' #500 ball was fished out of McCovey Cove by a city parks worker who, as far as I can tell from my cursory google-search, still has it today.
Any price tag put on the Mays' 500th HR ball is, of course speculative.
Some examples of public 500 HR ball sales are: Eddie Murray's 500th went for $280,000 in 1996. In 2007, Mickey Mantle's 500th sold for $144,000. The same year, Alex Rodriguez' milestone 500 ball sold for $105,000. Also that year, the ball Frank Thomas hit for his 500th was returned to him by a fan, as was Ken Griffey Jr.'s #500 in 2004.