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]
(..) bone-density scanner. Thank you for the opportunity to discuss it with you. - I appreciate it. - We just don't need it, Chris. It's unnecessary and expensive. - Well, maybe next... - Thank you. It gave a slightly denser picture than an x-ray for twice the money. - Hey. (..)2006 The Pursuit of Happyness