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.