It doesn't appear that he wrote it, however many sources say he did say it, or something very similar at a sci fi convention in 1948.
In 1948 he described a method which could solve his financial
problems. He has been quoted as telling a science fiction convention
in 1948: "Writing for a penny a word is ridiculous. If a man really
wants to make a million dollars, the best way would be to start his
-Methvin, Eugene H. (May 1990). "Scientology: Anatomy of a Frightening Cult". Reader's Digest. pp. 16. According to this.
Numerous people report the same or similar statement at the time:
Response to a question from the audience during a meeting of the
Eastern Science Fiction Association on (7 November 1948), as quoted in
a 1994 affidavit by Sam Moskowitz.
This statement is similar or identical to several statements Hubbard is
reported to have made to various individuals or groups in the 1940s.
The incident is stamped indelibly in my mind because of one statement that
Ron Hubbard made. What led him to say what he did I can't recall — but
in so many words Hubbard said: "I'd like to start a religion. That's
where the money is!" L. Ron Hubbard to Lloyd A. Eshbach, in 1949; as
quoted by Eshbach in his autobiography Over My Shoulder: Reflections
On A Science Fiction Era (1983) ISBN 1-880418-11-8
Y'know, we're all
wasting our time writing this hack science fiction! You wanta make
real money, you gotta start a religion! As reported to Mike Jittlov by
Theodore Sturgeon as a statement Hubbard made while at the Los Angeles
Science Fantasy Society clubhouse in the 1940s.
Writing for a penny a
word is ridiculous. If a man really wanted to make a million dollars,
the best way to do it would be start his own religion. As quoted in
the Los Angeles Times (27 August 1978)
Writing for a penny a word is
ridiculous. If a man really wants to make a million dollars, the best
way would be to start his own religion. As quoted in the article
"Scientology: Anatomy of a Frightening Cult" by Eugene H. Methvin.
Reader's Digest (May 1980)
I always knew he was exceedingly anxious to
hit big money — he used to say he thought the best way to do it would
be to start a cult. Sam Merwin, Editor of Thrilling Science Fiction
magazine Winter of 1946-47; quoted in Bare-Faced Messiah, The True
Story of L. Ron Hubbard (1987) by Russell Miller
Whenever he was
talking about being hard up he often used to say that he thought the
easiest way to make money would be to start a religion. Neison Himmel,
briefly a roommate of Hubbard in Pasadena during the fall of 1945, in
a 1986 interview, quoted in Bare-Faced Messiah, The True Story of L.
Ron Hubbard (1987) by Russell Miller