( 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
IN FOR A PENNY, IN FOR A POUND
Meaning of IN FOR A PENNY, IN FOR A POUND in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005