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]
(..) Oh, a diamond ring. And do buy a great big one, Rhett. You shall have the biggest and the most vulgar ring in Atlanta. I'll take you to New Orleans for the most expensive honeymoon... ...my ill-gotten gains can buy. Oh, that would be just heavenly. And I think I'll buy your trousseau for you, too. (..)1939 Gone with the Wind