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]
(..) In fact, it might be the only ship that's sailed around here lately. That damn war in the Philippines has interrupted my supply of narcotics. They're prohibitively expensive if they can be found at all. I venture to guess that's the reason for your coming here today? What's in this stuff anyway? (..)2014 TvShow: The Knick Title: The Golden Lotus Season: 1 Episode 9