Hubbard starting a religion on a bet is an urban legend. The following is excerpted from
http://www.urbanlegends.com/religion/hubbard_heinlein_bet.html
"The way to make a million dollars is to start a religion." L. Ron Hubbard is widely rumored to have said "The way to make a million dollars is to start a religion." There are also variant rumors. For some reason, this is often mentioned on Usenet. The evidence is discussed below, but the short answer is that several reputable people claim to have heard Hubbard say it.
One form of the rumor is that L. Ron Hubbard made a bar bet with Bob Heinlein. This is definitely not true: it's uncharacteristic of Heinlein, and there's no supporting evidence. There is, however, inconclusive evidence that Bob Heinlein suggested some parts of the original "Dianetics".
Another claim is that George Orwell said it first. This is true: in 1938, Orwell wrote "But I have always thought there might be a lot of cash in starting a new religion...". However, he wrote that in a private letter, and I don't know the first publication date. If Hubbard did get the idea from Orwell, that would merely make the "joke" unoriginal.