Here's how you KNOW you're rich -- when you drop $3 mil on a baseball that later plummets in value, AND YOU JUST DON'T CARE!

Case in point ... Todd McFarlane, who's explaining to TMZ Sports why he's not remotely bothered by the fact his Mark McGwire #70 home run baseball is only worth a fraction of what he paid for it.

Remember, Todd -- one of the most successful comic book artists of all time -- bought the record-breaking ball in '99, right after McGwire and Sammy Sosa wrapped up that famous home run chase.

Demand was high at the time -- but the steroid scandal (and Barry Bonds) caused the value to sink like a stone. Multiple experts say the ball is only worth around $300k to $400k these days ... if that.

So, we hit up Todd ... and wow, HE IS UNFAZED!!!

"If you look at it as buying a baseball, dollar in and dollar out, I'm the stupidest guy on the planet. It's a 3 dollar baseball. I overpaid by a million times that amount right so it doesn't make any sense," McFarlane says casually.

"Even as a collectible and again at this point, I didn't know the cloud of steroids were coming. I thought, in best case scenario, I could use it and then I can sell it and get my money back but I'd get all the value of in maybe being able to sell some sports figures."

"[But] even if I get $0 for it, I've made that money back 5 to 10 times over in the last 15 years."

How???

Long story short ... Todd says owning the baseball has unlocked all sorts of doors in the business world just from people curious to see it ... or just meet him.

Todd says he's been able to make a fortune from those deals -- so at the end of the day he's happy with the purchase.

"I keep saying to people, it was my ante to the poker game with the big boys."

"I bought that ball in 1999, it is now 2020 -- 21 years later you and I are still talking about it. Yesterday ESPN was giving me national attention. Every time we talk about that ball, I get to amortize it even further than I've already done it. So, I don't care if I get $20 for that ball. I'm clean on this deal."

When asked if there was a number that someone would offer to take the ball off his hands, McFarlane had a baller answer for that too!

"I don't know. It would have to be a stupid number that I don't think anybody would give and here is why I say that. Never take money that you don't need, right? So, if my businesses are hurting and somebody came in and gave me a low ball offer and I needed some investment capital or my kids were sick and I needed to pay some medical bills or whatever it was right? Even if I wanna buy a Maserati or something stupid, then I might take it. But as I sit here today, I don't know what I would do with the money regardless. It would have to be a dumb one because I'm better off to hold it."

We also asked Todd if there was a dream collectible he'd love to buy ... and you gotta hear his answer.