The budget (film-making, marketing, everything - which is what a budget is) approached $500M. The main film-makers on projects make the most of their money with percentages known as "back end", rather than their fees: see me. A deferred fee for Cameron was made up tenfold in his back end.

Cameron can do as he pleases, and has been for years.

By the way, the production of Avatar was four years. He came up with the idea 14 years ago, wrote the script years later - the technology wasn't there - the film did not take a long time to make.