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 ... if Brady plays a full 17-game season in 2022 -- plus the playoffs -- he's expected to throw roughly 30-to-50 more touchdown passes.

Which, of course, would mean the last TD he threw to Mike Evans in the Bucs' playoff game against the Rams in January is significantly less valuable.

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.