At one time, a movie budget of a million dollars made Hollywood cringe, but that was a lot of inflation ago. Still, movie budgets have far outpaced the rate of inflation for several reasons. These reasons came together to get us where we are today: fewer movies with bigger budgets. That makes sense as long as you are betting on a sure thing.
Avengers: Infinity War – Part 1 and Avengers: Infinity War – Part 2 are the follow-ups to The Avengers and Avengers: Age of Ultron, which are scheduled for release in 2018. Like their name suggests, they will be making use of plot elements that have been brought up in previous entries in the Marvel Cinematic Universe, meaning that in a real sense, they are meant to be a sort of climax that the series of movies have been building towards. As a result, it is no wonder that a lot of people are prepared to believe that the two movies will have a combined production budget of $1 billion.
Even if true, that would be for two movies. But we are getting closer to the day that a billion can be spent on a "sure thing" in Hollywood (like maybe a Marvel superhero movie starring Dwayne "The Rock" Johnson directed by James Cameron). Read about how and why movie budgets are ballooning beyond all reason at TVOM.
I don't think Cameron movie get an official budget anymore. With Avatar Fox just asked him to try not too crazy.