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 hoop earrings were her thing and that I wasn't allowed to wear them anymore. And then for my Hanukkah, my parents got me this pair of really expensive white-gold hoops. And I had to pretend like I didn't even like them, and it was so sad. And you know she cheats on Aaron? (..)2004 Mean Girls