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]
(..) - Nice to meet you. Pleasure meeting you. You have a beautiful house. Thank you. Pool's a little cold, but it's very expensive to heat, so... I think I'm gonna goin and check the tide tables 'cause I'm gonna take the boat out early in the morning. - You should come. (..)2011 Friends with Benefits