“When a habit begins to cost money, it is called a hobby” is a jocular one-line saying that appears to have been coined in the “Little Liz” newspaper comic strip in January 1958. “Hobby” sounds better than “habit” and provides a somewhat better cover for losing money.
 
 
3 January 1958, Mexia (TX) Daily News, “Little Liz” comic strip, pg. 4, col. 5:
When a habit begins to cost money it’s called a hobby.
(Someone is shown painting—ed.)
   
Google News Archive
24 September 1964, The Spokesman-Review (Spokane, WA), “Shop with Sue,” pg. 3, col. 6 ad:
When a habit begins to cost money it is called a hobby.
 
Google Books
Quote Unquote
By Lloyd Cory
Wheaton, IL: Victor Books
1977
Pg. 153:
When a habit begins to cost money, it is called a hobby. (Supervision)
 
Google News Archive
13 January 1989, Idahonian/Daily News (Moscow, ID), “Tell it to Ann” by Ann Landers, pg. 11A, col. 8:
My Laugh for the Day: When a habit begins to cost money it is called a hobby.
   
Google Books
14,000 Quips & Quotes:
For Speakers, Writers, Editors, Preachers, and Teachers
By E. C. McKenzie
Grand Rapids, MI: Baker Book House
1990, ©1980
Pg. 225:
When a habit begins to cost money, it is called a hobby.
   
Illustrated World of Proverbs
5/19/12
When a habit begins to cost money, it’s called a hobby.
~ Jewish Proverb
 
The Wealth Guide
June 29. 2012
(...)
To me, I believe in striking balance. While I work hard to get out of rat race, I reward myself with my hobbies. My greatest hobby is traveling. (When a habit begins to cost money, it’s called a hobby.)