$15 million budget?!

Multiple sources are reporting that this film only had a budget of $15 million. I saw the film yesterday and loved it and am astounded that it only cost $15 million to make. It's scope and scale, and amount of effects, would seem to preclude a budget anywhere close to this low. I feel like if Hollywood made this very same film it would cost $90 million.

If it's possible to make a movie like this on such a small budget I'm surprised we don't see more independent low-budget, high-concept action movies.