According to CBR, in order for the movie to break even is $600 million when everything is factored in. Production budget ($250 million, plus $25 million in reshoots, then there's the millions spent on promotion costs).
CBR is way underestimating marketing costs. These days the biggest blockbusters easily spend $100 million or more in promoting the movie. So Justice League will at a minimum have to make $700 million to break even ($250m budget + $100m marketing = $350m x 2 since theaters get roughly half the share).
Which seems right, given that I remember the Avengers needing to make something near a billion dollars, maybe a few hundred million lower, to cover costs and then turn into profit.
Big movies are high risk games when they do not work out.