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 ran guns to Ethiopia. - You fought the fascists in Spain. - What of it? Isn't it strange you always seem to fight with the underdog? I found that an expensive hobby. But then I never was much of a businessman. Are you enough of a businessman to appreciate francs? I appreciate it, (..)1942 Casablanca
(..) We don't want our children to grow up there. So you decided to go to America. But we have not much money... ...and traveling is so expensive and difficult. It was more than we thought to get here. Then Capt. Renault sees us. He's so kind. He wants to help us. (..)1942 Casablanca