Production began September 25th for the 4 AVATAR sequels. The total budget is set to exceed $1 billion.$1 billion is definitely a lot of money but with 4 sequels in the works, the budget averages out to about $250 million per film. For context, Captain America: Civil War, Batman v Superman: Dawn of Justice, and Star Wars: The Force Awakens all had budgets around that point.
It isn’t rare for Hollywood blockbusters to go for high budgets when they expect to gross even higher. And with James Cameron’s AVATAR sitting as the highest grossing film of all time (not adjusted for inflation; when adjusted for inflation it sits at #2 behind Gone With the Wind), it isn’t surprising the studio is willing to give Cameron the money he needs to make 4 more highly profitable blockbusters.
We do not know much about the plots of the upcoming films but the antagonist in all 4 will be a returning character from the first film. Cameron also intends for the films to be 3-D but without the glasses. It looks like Sigourney Weaver will be returning as well. We also have the release schedule:
Avatar 2 is set to release December 18, 2020, and the third film is set to release December 17, 2021. Taking a 3 year break, the fourth installment will release December 20, 2024, and the final installment will release December 19, 2025.