http://regressing.deadspin.com/how-much-is-lebron-really-wor...
http://www.sbnation.com/lookit/2013/5/1/4291088/lebron-james...
And many more...
It is generally agreed upon that in a salary capped league, and especially in basketball wherein a single player has a disproportionate impact on the outcome of a game, salaries are depressed in comparison to what teams would pay for his services on an open market. Lebron guarantees a competitive team no matter where he plays and significantly increased revenues and franchise values when leaving for Miami and returning to Cleveland.
The players also have a strong union, which negotiates a number of protections for the players, from minimum salaries to safe work conditions, while allowing for higher compensation for key players.
So it is very reasonable for those of us on the left to be against the extreme labor exploitation represented by the salary cap.
[1]http://fivethirtyeight.com/features/kawhi-leonard-like-all-t...