http://collider.com/justice-league-runtime-budget-revealed/
The filming of Justice League was a unique process. When Geoff Johns and Jon Berg were hired to oversee the DC movie slate and give it a more optimistic tone, their first job was to get with Zack Snyder and screenwriter Chris Terrio to rework Justice League and make it a more hopeful movie. Per WSJ, when Snyder delivered his cut earlier this year, studio executives “felt he had made progress with the lighter elements,” but also felt the movie could use more work.
That’s when Joss Whedon was brought in to write scenes for the reshoots, and when Snyder left after his daughter’s death, Whedon directed and oversaw what WSJ describes as “significant” reshoots that brought the budget up to $300 million.