Say the conspiracy is correct, and there are a select few at NASA who know the truth of the earth being flat, surely they would be so high up within NASAs ranks that they would already command a substantial salary which would entitle them a powerful, rich and lavish lifestyle.
If they were skimming the money to increase their own personal wealth it would be an exercise in futility, they wouldnt be able to justify spending it on anything without the proper authorities, including auditors, picking up on it.
So to come back to the OP, where is the motive?