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.
The reasoning is obvious .