Tom Brady's unretirement just cost a collector a small fortune ... with one auction expert telling TMZ Sports the QB's "final" TD ball is now worth a mere $50K -- after it sold for over $500K just three days ago.
Our auction source tells us with Brady coming back to the NFL ... the ball that was just auctioned off on Saturday is now worth roughly a 10th of what it just fetched at auction.
In fact, our source tells us they wouldn't be surprised if the ball was actually worth closer to $25K than $50K at this point.
There is good news for the buyer, however ... our source says it's probably unlikely they'll have to fork over the half mil for the ball.
Our source tells us the ball was advertised as Brady's last TD ever -- and now, since it won't be, the buyer could have a legitimate claim to renege on the deal. Our source also says it's likely the buyer is a whale in the auction world, and the auction house that sold the piece would almost certainly fight to keep him or her happy.
No word yet on who bought the ball ... but whoever they are and wherever they are, we're sure they're sweating right now regardless.