According to Forbes, LeBron James is responsible for the biggest gain in team value, with the value of the Heat going up 26% upon his arrival. He's also responsible for biggest drop, with Cavs' worth falling 17%.
Here's an excerpt from the article, which also addresses NBA team values generally:
The average NBA team is now worth $369 million, 1% more than last year. But several of the league’s 30 teams have not fully recovered from the recession, and as a result values are still 2.6% below the $379 million peak average they hit two years ago.
No player in the 64-year history of the National Basketball Association has come close to having the immediate and profound impact that LeBron James had when he bolted the Cleveland Cavaliers for the Miami Heat this summer. The King’s move accounted for both the biggest gain and drop in team values: the Heat’s worth increased 17%, to $425 million (seventh among the NBA’s 30 teams), and the Cavaliers plummeted 26%, to $355 million (15th).