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]
(..) - One, two... - Where you going? - I gotta go pee. - Anastasia. - Yep, this is me. Sending back your expensive books, cause I already have hard copies of that, Thanks though, for the kind gesture. You're welcome. Where are you? Oh… I'm in line, cause I have to pee really bad. (..)2015 Fifty Shades of Grey