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]
(..) They've sent us Harry's as well. Dumbledore must know you're here. Doesn't miss a trick, that man. -No. -This lot won't come cheap. -The spell books alone are expensive. -We'll manage. There's only one place we're going to get all of this. Diagon Alley. Right. Here we are, Harry. (..)2002 Harry Potter and the Chamber of Secrets