It's a good thing Todd McFarlane's rich ... because the Mark McGwire baseball he bought for $3 MIL back in the day has depreciated LIKE CRAZY!!!
Like, not even worth $500k ... according to some of the top sports memorabilia auction houses in the world.
Todd -- one of the most famous comic book artists ever -- famously purchased Mark's 70th HR ball at an auction in '98 ... right after the end of the '98 season.
The final gavel price was around $2.7 -- but after taxes and fees, Todd says he dropped "a hair over $3 million."
In fact, Todd showed off the ball as part of his amazing collection in the opening scene of ESPN's 30 for 30, "Long Gone Summer."
"This is the big boy," Todd said ... "This is the 70. [It's] been scuffed up a little bit 'cause it got hit 400 feet."
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.