Adrien Brody Drops $20,000 ... On Monopoly Man

There's a reason rich people buy expensive art ... they've run out of everything else to buy -- which is exactly how Adrien Brody blew $20,000 this week.

Sources tell TMZ, the actor stopped by a popular Hollywood art gallery called Lab Art on Tuesday to check out new works by famed street artist Alec Monopoly ... and he was so blown away by the paintings, he purchased 5 pieces worth roughly $20,000 total.

Adrien went to the gallery to check on his old Rolls-Royce which he donated to Alec, who turned it into an art piece ... and Adrien went nuts over several paintings.

Check out the Rolls in the pics. Pretty cool actually.