Remember, in '98 the home run chase withSammy Sosa was the biggest sports storyline in America. There was tremendous interest in any memorabilia from the HR race. Demand was high!
But, in the years that followed, Barry Bonds broke McGwire's record and Mac later admitted he was on steroids that season anyway. Demand plummeted.
Not good for long-term value ... so says our experts.
Ken Goldin of Goldin Auctions now estimates the ball's current value at $250k to $300k.
David Kohler, who runs SCP Auctions, is a little more optimistic ... "$250,000 to $400,000, tops."
These guys know the business ... Ken's been dealing with high-end collectibles for decades and recently sold a rare Mike Troutbaseball card for more than $900,000.
David specializes in game-used historic baseballs ... selling some of the most famous HR balls ever including Barry Bonds 762 ball, A-Rod's 500 and 600 HR baseballs and more.
In other words, they know the market.
McFarlane has said over the years that he isn't too bothered by the steroid controversy ... insisting the ball will always be a piece of sports history and therefore, valuable.
Todd also happens to own the famous Barry Bonds #73 ball from the 2003 season, which he reportedly bought for $500k.
We've been told the ESPN doc has already generated some interest in Mark McGwire and Sosa items (just like "The Last Dance" didwith Michael Jordan stuff) ... but it's pretty unlikely McFarlane will ever get his investment back on that McGwire ball.