“Why buy the cow when you can get the milk for free?”

“Why buy the cow when you can get the milk for free?” (or, “Why buy a cow when the milk’s so cheap?”) means that you needn’t buy something when it’s being offered cheaply or even for free. The adage most frequently has applied to marriage, but can also apply to free/pay internet websites. If a woman freely has sex with a man, the adage popularly intends, why should he marry her?
   
The adage (which dates to at least 1890) sometimes is used in milk advertisements to show a low price.
   
   
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Why buy the cow when you can get the milk for free?
(idea) by enkidu Thu Apr 12 2001 at 0:42:18
Generally speaking, why make a long-term investment when the benefits are available without it? The image it brings to mind is of a cow, which requires care, feeding, and shelter, and in return produces milk. Why would anyone go to the trouble and expense of owning the cow, if they could get milk without it?
 
The metaphor is usually applied to sex, from a male perspective: Why enter a monogamous or committed relationship with a woman, as long as she’ll keep having sex with you without it?
   
Google Books
August 1890, The Arena, pg. 362:
My mother did not hesitate to openly declare that Aunt Helen had been deserted by numerous lovers because (to quote my mother) “why should they buy a cow when milk was so cheap?”
 
16 October 1905, Atlanta (GA) Constitution, pg. 7, col. 4:
The Milk is Free.
(From The Camilla Citizen.)
Some claim that Hoke Smith does not control The Journal, and has no personal organ. He would be a fool to buy a cow when milk is free. Why should he want an interest in a paper that seems to place him on a pedestal of virtue unequaled since St. Paul?
   
Google Books
Proverbs of the Pennsylvania Germans
By Edwin Miller Fogel
Published by The Pennsylvania-German Society
1929
Pg. 118:
Why buy a cow when you can get milk at ten cents a quart (ie why marry, etc., etc.)?
   
6 May 1939, Burlington (NC) Daily Times-News, “On Broadway” by Walter Winchell, pg. 4, col. 3:
In Jack White’s: “Support her? I should say not! Why buy the cow when the milk is so cheap?”
     
Google Books
The Outlander
By Germaine Guèvremont
Published by Whittlesey House
1950    
Pg. 208:
“What did you want to marry for? Why buy the cow when you can get the milk for nothing?”
     
Google Books
Tooner Schooner
By Mary Lasswell
Boston, MA: Houghton Mifflin
1953
Pg. 110:
“But why should he want to marry her? Why buy the cow when you’re getting the milk free?” 
 
31 May 1956, Hope (AR) Star, pg. 3, col. 1 ad:
WHY BUY A COW WHEN MILK IS THIS CHEAP
PET MILK
Tall Cans
2 Cans 25c
(Williams Grocery and Market—ed.)
     
Google Books
The Circle Home
By Edward Hoagland
New York, NY: Crowell
1960
Pg. 153:
“Why buy the cow when milk’s so cheap is what you were afraid of,” Kelly finished. 
 
Google Books
Proverbs and proverbial phrases of Illinois
By Frances M. Barbour
Published by Southern Illinois University Press
1965
Pg. 41:
Why buy the cow when you can get milk free?
   
Google Books
Don’t Marry That Woman! Or, Why Buy a Cow, When Milk Is So Cheap?
By Ira Lunan Ferguson
Published by Lunan-Ferguson Library
1973
     
New York (NY) Times
TV TURNS TO THE HARD-BOILED MALE
By PETER J. BOYER
Published: February 16, 1986
(...)
A typical exchange between them occurred over a discussion of a long-unsolved murder, in which a woman and her lover went to the electric chair, accusing each other of murdering the woman’s husband:
 
Maddie: ‘‘Addison, clearly he had more reasons to kill him than she did.’’
Addison: ‘‘Name one.’’
Maddie: ‘‘Her. He wanted her. He had to kill her husband to have her.’’ 
Addison: ‘‘That’s crazy! Why buy the cow when you can get the milk for free?’‘
   
Google Books
Why Buy the Cow
By Ella Shines Goldsmith
Published by Trafford Publishing
2003
 
Google Books
Why Buy the Cow
By Subrah S. Iyar
Published by Lulu.com
2007
 
Google Books
Why Buy the Cow?: A Unit on Cohabitation
By Jessica Pool
Published by The Dibble Institute for Marriage Education
2008
     
New York (NY) Times
Q & A with Stuart Elliott
Published: March 9, 2009
(...)
Two recent (and still running) commercials that jump to mind are for Verizon mobile phone service. One Verizon spot has a woman running into an old flame and marveling that he had now “committed” to Verizon when he couldn’t do the same for her.
 
In the original version of the ad he explains that he only pays for the minutes he uses and says, “Why buy the cow when you can get the milk for free?” She concludes that he hasn’t changed at all and then moves on.
 
That version ran only for a short period before the “cow” reference was edited out. I can only imagine that someone somewhere concluded that the line, spoken out loud on TV, was offensive.