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]
(..) ever again. I'm sorry. I guess we should cool it for a while. I'm facing a potentially very expensive divorce. No, no, I understand completely. It's... "In order to be successful, one must project an image of success at all times." (..)1999 American Beauty