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]
(..) That's worth something. Do you have someone who can drill for it? -Do you think there's oil here? -I know there is. It's very expensive to drill. To get it up and out of the ground. You ever tried that before? -How much is it? -Costly. Well, our oil sits right up (..)2007 There Will Be Blood