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.
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