This is a pretty popular rumor. It was allegedly uttered at the Eastern Science Fiction Association meeting of 7 Nov 1948. (Although, other reports have it in 1947, or in a bar, or in Lloyd Arthur Eshbach's hotel room.) Here's what we do know: In 1938, science-fiction writer George Orwell wrote in A Letter to Jack Common "But I have always thought there might be a lot of cash in starting a new religion...". In addition, both David A. Kyle and Jay Kay Klein both signed affidavits that the incident did not take place. Most of these claims are from decidedly anti-Hubbard books, including one co-written by Hubbard's estranged son.
Of course, my common sense deduction is that Hubbard would not create and dedicate his life to a religion for "a million dollars". And if this was the case, why did he give the great majority of his income from royalties from book sales back to the Church? This one doesn't make sense.
And quite honestly, even if it was said as a flip statement to a friend or associate, there was no indication at that time he planned to start a religion. There's no way to know the context it could have been said in, etc. It was 2 years before the release of the book Dianetics and Hubbard had no way of knowing the public reaction to the book, and his subsequent research, would push his life in that direction.
David A. Kyle affidavit
Jay Kay Klein affidavit
Second Affidavit of David A. Kyle