Coûter bonbon means to cost a lot. However, literally, it means “to cost a candy!”. The origin of the idiom is not very clear. Bonbon seems to be a doubling of bon. And coûter bon used to mean coûter cher (to be expensive). Coûter bonbon?…coûter très cher (to be very expensive).
Coûter bonbon
French with Valerie 