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]
(..) - It is me. Pete? One second. Don't do what Frankie and Johnny did. Don't be a moron with the money. Karen, Judy, Ruth, come here! I got the most expensive tree they had. - What do you think, Mommy? - I love them all. I love the gold outfit. - Merry Christmas. - Thank you. (..)1990 Goodfellas