To my knowledge he never admitted it was made up. He was however a sci-fi/fantasy novelist before he started the church of scientology, which makes many believe that he did it to manipulate people and get money. However, most reports of him, especially in his mid to late life describe him as a 100% believer, but also erratic, paranoid, and eccentric. I would suspect he believed what he taught, but had some mental issues going on.

At one point Hubbard allegedly (I believe it based on the sources and the evidence, but it is not like its a documented, objectively proveable fact, like most history) said that writting for pennies a word (pulp fiction) was dumb and that the real money was in starting a religion. Another (slightly less creditable) event would be that he might have made a bet with Heinlein that he could create a better religion. Heinlein supposedly wrote Stranger in a Strange Land while Hubbard supposedly created Dianetics and eventually Scientology. I do not believe that second one, but I DO believe he made the statement that "the real way to make a million bucks is the start a religion".

I would not say that is explicitly stating that the religion was invented for money. For one, the statement pre-dates the religion, for another, there is a difference between a statement made between friends and an express statement of intention and planned activity. My own personal take on the situation is that Hubbard created Dianetics and Scientology because his ideas were turned down by the APA and he was too narcissistic to admit that his "discoveries" were all wrong. So he went around the scientific community and started a religion, where everyone would hang on his every word as ultimate truth. I truly do not think he was in it for the money, I think he was in it for the glory and the personal validation brought by creating a cult of personality.