The Devil Wears Prada 2 will outgross the entire 20-year theatrical run of the original inside a month. The first film cost $40 million. The sequel cost $100 million. The math looks reckless until you read the budget breakdown. David Frankel, who directed both films, told the New York Times the sequel's $100 million budget "mostly went to [the cast]." Streep, Hathaway, Blunt, Tucci. The same four people the audience watched in 2006. Devil Wears Prada 2 opened to $233.6 million worldwide.…