$300M+$150M (not exact but what the OP's estimate is)=$450M
They would need to double that number to make a profit ($900M) and studios don't get all the money made especially overseas in places like China where they only get 25%. That's why Avengers 2 was considered a "disappointment" even though it's the 6th highest grossing film of all-time grossing $1.4B. And the production cost for that is less than Spectre (so imagine the pressure for this movie).
But since you didn't make it totally clear, the movie theatres get a big chunk of the receipts - about half for the US, more overseas. They need the money to cover operating costs.
(And then the concession stands for actual profits.)