“It’s only money” means that there are other things more important than money. The person who loses money can get it back again. A person’s health, for example, is always much more important than money.
 
The newspaper columnist “Ruth Cameron” (Persis Dwight Hannah) was syndicated in over 150 newspapers when she wrote “After All, It’s Only Money” in June 1914. Cameron wrote:
 
“Say to yourself, ‘after all, it’s only money. It isn’t death. I haven’t lost anything I can never get back.’ Think a moment of losing someone out of your life who makes up the half of it; or carry yourself back in memory to some chill moment of past loss, and see if you cannot say with more than lip sincerity, ‘after all, it’s only money.’”
 
“it’s Only Money” was the title of a song, with music by Jule Styne and lyrics by Sammy Cahn, in the movie Double Dynamite (1951). It’s Only Money (1962) was a movie that starred Jerry Lewis.
 
‘It’s only money” is often said by someone who is rich and who is not afraid to spend large amounts of money on desired items. To someone who has lost money and who has become poor, “it’s only money” is used in sad irony.
 
 
2 June 1914, Colorado Springs (CO) Gazette, pg. 4, col. 3:
“After All, It’s Only Money”
BY RUTH CAMERON
(...)
“It’s only money, after all”— do you ever remember to say that when you find yourself fretting over money affairs?
 
Try it some time.
 
Say to yourself, “after all, it’s only money. It isn’t death. I haven’t lost anything I can never get back.” Think a moment of losing someone out of your life who makes up the half of it; or carry yourself back in memory to some chill moment of past loss, and see if you cannot say with more than lip sincerity, “after all, it’s only money.”
 
Google News Archive
24 December 1930, Rochester Evening Journal and The Post Express (Rochester, NY), “Tiger Heart” by Robert Terry Shannon, pg. 9, col. 2:
“After all, it’s only money and nothing to worry about,” Mrs. Calvert said.
 
Google News Archive
20 November 1948, The Spokesman-Review (Spokane, WA), “Suspended Star Now Forgiven” by Sheilah Graham, pg. 5, col. 7:
Groucho Marx, when asked what part he plays in “It’s Only Money,” says, “I’d like to play the money!”
     
The Internet Movie Database
Soundtracks for
Double Dynamite (1951)
“It’s Only Money”
Music by Jule Styne
Lyrics Sammy Cahn
Sung by Frank Sinatra and Groucho Marx;
Reprised by Sinatra, Marx and Jane Russell
 
The Internet Movie Database
Plot Summary for
It’$ Only Money (1962)
Lester is a clumsy and awkward TV repair man who is nevertheless gifted technically. In helping out a friend, he is drawn into a mystery involving a missing heir in a rich family. He begins to notice little things, like how much those family portraits look like him. Surely..no..he can’t be…can he ? Written by David Gibson

   
Google Books
Shorter Dictionary of Catch Phrases:
From the Work of Eric Partridge and Paule Beale
Compiled by Rosalind Fergusson
New York, NY: Routledge
1994
Pg. 76:
it’s only money! a carefree, rueful or ironic attitude to expense or expenditure, as in it cost me a hundred quid, but what the hell? It’s only money! The phrase dates from around 1925.
   
Early Retirement Extreme
It’s only money…
Published on August 20th, 2012
Posted by Jacob
One thing you will hear if you adopt an extreme savings program is the popular refrain “It’s only money”, “There are more important things than money”, etc. I agree with the last statement, but it is NOT just money. It is what money represents that matters.
 
Money represents many things.
 
Money is time. It represents the time you have to work to get something you need. Unless you truly love your job, this is time you would have spent doing something else, time you will never get back. In other words, you can always get more money, but time is finite, we all die, so it would be better not spend too much of it working.