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]
(..) and I can't get a penny ahead. Sundance says it's because you're a soft touch and always taking expensive vacations, buying drinks for everyone, and you're a rotten gambler. That might have (..)1969 Butch Cassidy and the Sundance Kid