Tristan Thompson's one-finger salute just cost him a chunk of change ... 'cause the Cavs star's been slapped with a $15k fine for flipping off hecklers at the Nets game on Monday.
TMZ Sports posted the video ... T.T. snapped at the end of the Cavs-Nets game at Barclays Center ... flipping off some courtside fans after Cleveland sealed the W.
The league announced Thompson's punishment on Thursday ... and it's a bargain compared to the $25k fines handed to Kevin Durant and Patrick Beverley for their recent fan incidents.
It's the only recent blemish in Thompson's hot streak ... as the dude is averaging a double-double so far this season and has been helping the Cavs keep from being a complete embarrassment.
Let's keep it a buck -- T.T. is in the middle of an $82 million contract, so he's not even blinking.