After Shaquille O'Neal left the Orlando Magic in the summer of 1996 for Los Angeles, the NBA gave teams a better chance of retaining their stars by giving them an advantage. They were allowed to give an additional year and higher raises. However, in reality, this only entices free agent who are only going to sign one max contract in their career. That's why Cleveland can not really offer LeBron 30 million dollars more.
What the Cavs can offer LeBron is a six year deal for 125 million dollars. Without a sign and trade, other teams can only offer five years at 97 million dollars. Let's just focus on the first five years. In the first year, LeBron would be paid about 16.5 million. In each successive year, the Cavs will give him 10.5% raises. That means that in Year 2 he makes 18.23 million, he makes 20.15 million, then 22.26 million, and 24.60 million in the fifth year.
Teams, like the Knicks, Bulls, Heat, Nets, and Clippers, can only give LeBron 8% raises. That means that he will make the a similar amount in year one. The successive years' salaries go as follows: 17.82 million, 19.25 million, 20.79 million, and 22.45 million.
Over the first five years, the difference between the Cleveland contract and a contract from another suitor is a little over five million.
In the sixth year of the Cleveland deal, LeBron stands to make 27.18 million.
It's not a stretch to think LeBron James will sign another max contract or an extension that will increase his salary. If he signs a second max contract, he will make 105% of the previous years salary in the first year of the new deal. So that would mean that his second max (if he signs with a team other than Cleveland) would have a first year salary of 23.60 million dollars.
That means that over the course of the six years LeBron stands to make a little more than 8.5 million dollars, not 30 million. That's also not assuming bonuses that could be attained by winning championships or making the Finals (which won't happen in Cleveland).
Though the NBA tried to give an advantage to teams who are attempting to retain their stars, the advantage is greatly reduced if the player is too good. Players like Bosh, Amare, Johnson, or even Wade (injury issues and possibly breaking down six years from now) are only going to sign one max deal and this advantage gives their teams a bigger advantage in negotiation. However, for a guy like James, his skill destroys Cleveland's ability to pay him 30 million more than any other team.