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]
(..) you can just read what you need. No? OK, my turn. I see someone who's waiting for their first scoop, so their editor will notice them. You're wearing an expensive skirt, that's been rehemmed twice. Only posh skirt you've got. Your nails, you can't afford to do them that often. (..)2012 TvShow: Sherlock Title: The Reichenbach Fall Season: 2 Episode 3