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]
(..) But, yes, your debts are considerable. And you're now engaged in a conflict on several fronts. We both know how expensive war can be. And we both know gold wins wars. Your vaults are empty. Your late husband's profligacy saw to that. Your wealthiest allies, the (..)2017 TvShow: Game of Thrones Title: The Queen's Justice Season: 7 Episode 3