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.