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]
(..) This must've cost him a fortune. I can't believe it. Remember when he fell in love with Carol... ...and bought her that expensive crystal duck? What did you just say? Crystal duck. No, no, no. The "love" part? -Oh, my God! -Oh, no, no, no. (..)1995 TvShow: Friends Title: The One Where Rachel Finds Out Season: 1 Episode 24