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]
(..) l just wanna go home. Spike, see who that is, and put some clothes on, for God's sake. Looks like a chauffeur to me. Spike owes you an expensive dinner or holiday, depending who's got the brains to get the going rate on betrayal. That is not true. Wait a minute. This is crazy behaviour. (..)1999 Notting Hill