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]
(..) a time machine. - They do? - Yeah. - Are-Are-Are you sure? - Can it get me back to ? - Oh, yeah, but it's, like, really expensive. And it breaks all the time 'cause some smart guy made it a long time ago. I don't care. Just get me there, okay? - Well... (..)2006 Idiocracy