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Phrase(s): Why buy a cow when you can get milk for free? [and] Why buy a cow when milk is so cheap?
Prov. Why pay for something that you can get for free otherwise. (Sometimes used to describe someone who will not marry because sex without any commitment is so easy to obtain. Jocular and crude.) • I don’t have a car because someone always gives me a ride to work. Why buy a cow when you can get milk for free? • Mary told her daughter, "You may think that boy will marry you because you’re willing to sleep with him, but why should he buy a cow if he can get milk for free?"
McGrawhill's American Idioms And Phrasal Verbs