MJ got 35 in 1998. LeBron isn't MJ but in terms of earning power, economic potential he isn't that far behind. That was in 1998 when the salary cap was about 60% of what it is now.
Fair Market Value for a Kobe/LeBron type superstar in their prime to an NBA team (stars are verything in the NBA) is probably between 45-55mil depending on the city.