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]
(..) - Yeah. Hey, Travis, this here's Easy Andy. He's a traveling salesman. How you doing, Travis? - You got a. magnum? - It's an expensive weapon. That's all right. I got money. It's a real monster. It will stop a car at yards. Put a round right through the engine block. Here you go. (..)1976 Taxi Driver