The film's producer (Tom Pollock) reports that their agreement with Fox Searchlight Pictures was that they would stick to a budget of approximately $15M.
Finally, Fox Searchlight Pictures signed on ― with some conditions.
“We had to get the budget down to $15m-something and we had to source
half, because that’s how studios operate today,” says Pollock. “Ivan
Reitman and I put up the money ourselves through Cold Spring Pictures,
through which we have previously invested in Up In The Air and No
Strings Attached.” - Screendaily interview
The movie itself was filmed over 35 days, implying that more than 70-80% of their total budget was spent on "below the line" costs such as filming (cinematography/lights), talent costs (actors), effects/makeup and post-editing.