As the old saying goes, don't ask a man who makes chicken sandwiches how much money it costs to make a movie. He's got no fucking clue! Nevertheless, here we are in a day and age when anything seems plausible, so news sources pick it up and run with it like madmen. As of press time, no less credible sources than Newsarama and Cinema Blend have already reported on this as if its gospel truth, so allow me to help you figure out why people seem to think this carries water.
The original article that both of those sites sourced was published on the site Atlanta Business Journal—always my first source for news—and the quote that article published that was glommed onto by the big dogs was from none other than Chick-Fil-A founder and noted homophobe Dan Cathy. While speaking at an African American Film Critics Association luncheon on February 28, Cathy said the following in reference to the Pinewood Atlanta Studios where Marvel shoots all of its films...
"We now have on the lot down there now the largest film production ever with a $1 billion budget," Cathy said.
Now granted, Cathy is the owner of the Pinewood Atlanta Studios and likely has some grasp on what's happening on his lot, but just because he says something like this at a luncheon doesn't mean its any more true than if he said, "I have lots of gay friends." Oh wait, he did say that.
The larger point is that there is no way on (Dan Cathy's capital G) God's green earth Marvel is shelling out a billion dollars to make a movie. If he was potentially referring to the production and p&a costs for both Avengers films, he's probably closer to the mark: 500 million to make the two films and another 500 million to market them. The point is, folks, you've got to check your sources, and if your source is the guy who became a billionaire making chicken sandwiches, he's probably not reliable when it comes to matters of motion picture budgets.