Sure. In free agency, you should expect to pay 3% of the league salary cap to get 1 Win Share. This year, 3% of the league salary cap is $4.1M. At his $17M salary, Barnes would be at market value if he produced 4.1 Win Shares. He is currently on pace for 5.1 Win Shares, which would be worth $21M. Thus, he is statistically outproducing his contract.
Barnes came into this season as obviously the 5th option on that starting team, and given the presence of Monk, with the expectation of being at best the #6 scorer on the team. He is currently just that in per-game average (#3-#6 are separated by 1.1 points) and #4 in total points, because he doesn't miss games. At .632 he holds the best TS% of any non-post player on the team (yes, he's more efficient - though more selective - than Fox). Among regular rotation players he has the third best FTr (behind Fox/Domas) despite having an extremely low usage rate (only marginally higher than Duarte's usage).
Fox and Domas are our stars. But even then, Barnes is being asked to take a back seat to Murray, and to Huerter, and to Monk and he goes out there every night and plays the fifth wheel, never complains, and despite being an afterthought in the offense still manages to statistically outperform his contract. His biggest sin is not being a flashy player, and being the sixth option on the team which leads to him being dogpiled for "not doing anything". I'm not saying we should declare him our lord and savior but the dude is holding his own and he gets nothing but crap for it.