Her decision to leave the company proved to be an 'expensive' mistake. [=a mistake that caused her to lose a lot of money]
They live in an 'expensive' neighborhood. [=a neighborhood in which houses, apartments, etc., cost a lot to buy or rent]
an 'expensive' shop [=a shop that sells expensive things]
(..) You're really surprised? Jacqueline's a lot younger than Miranda. She has a fresher take on things. Not to mention American Runway's one of the most expensive books in the business. Jacqueline does the same thing for a lot less money. And Irv- Irv's a businessman, you know. (..)2006 The Devil Wears Prada