The guy who gave back Tom Brady's 600th TD pass ball might have cost himself a small fortune ... 'cause one memorabilia expert says the pigskin was worth at least $500K!!
Here's the deal ... after catching the record-setting touchdown pass from Brady in the 1st quarter of the Bucs' win over the Bears, Mike Evans ran over to a Tampa Bay fan and gave away the ball.
Evans didn't think much of it at the time -- he does the nice gesture after most of his TDs -- but when he got back to the sideline, he realized his blunder.