Comic book movies are famously expensive to produce, partly due to the extensive visual effects needed to bring superheroes to life.
Snyder was doing a ton of world building in the movie, with underwater scenes in Atlantis, and Cyborg’s CGI appearance.
And since it also looks like the Snyder Cut will be a particularly long product, there’s a ton of work to do before it finally arrives.
Considering how many high concepts and over the top action that Zack Snyder had in store for Justice League, it make sense that it’s going to be an extremely pricy investment for HBO Max.