There is a thread with a (deeply flawed) study about the costs of a possible conspiracy, but there is no way it is even remotely accurate. The cost estimate is off by at least a couple orders of magnitude, and if NASA is so inept as they keep claiming, how could they keep such a huge secret?
The various space agencies have a combined budged of under 20 billion, and the bribes alone would eat up most if not all of that. Operating a functioning space agency and launching rockets into space and employing thousands of people is a huge expense.
There would be no money left to make enough of a profit to make such an expense worth it. In fact, a conspiracy would be deeply in debt.