in for a penny, in for a pound

Từ điển Oxford Advanced Learner 8th

ˌin for a ˈpenny, ˌin for a ˈpound idiom
(BrE, saying)used to say that since you have started to do sth, it is worth spending as much time or money as you need to in order to complete it
Main entry: ↑penny_idiom