https://www.basketball-reference.com/players/e/exumda01.html#advanced::none10.6 was the maximum he could have made with incentives. 1 million of those were "unlikely" so his base salary was 9.6 (the salary that shows up on the cap sheet). He missed 500k in likely incentives due to games missed, which I think was 500K.
https://www.spotrac.com/nba/utah-jazz/dante-exum-15357/
And you are just flat out undervaluing his defense.
I think his defense is good but overrated, and his offense is atrocious. The NET.
He has never had a positive +/- for a season. He has never had positive value over a replacement player.