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 do understand I'm your boss, right? Uh-huh. All right, let's ask the very, very expensive question. Is the probe going to be ready on time? Give me a number. All right, let's create days. (..)2015 The Martian