There is a currency exchange in the Mission Control menu. You can exchange either Earth currency or Moon currency for Mega Bucks. What is the point of this?
In the release notes (as of the last update, when it was implemented):
Convert your Earth and Moon Dollars into Mega-bucks. What does it do? That’s a secret! (Coming Soon)
So it isn't released yet, but it should have a purpose in one of the forthcoming updates.