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]
(..) Your favourite song: "Salsiccia's Own". I'm having a salsiccia sandwich. I'll be back. Hello, Kay. Congratulations, Michael. That's quite an honour. But a little expensive, wouldn't you say? - Cake? - No, thank you. - It's been a long time, hasn't it? - Yeah. - Eight years. - Yeah. (..)1990 The Godfather Part III