I suggest a trading system, when you, for example, could trade cash for tokens.
Here are my suggestions:
15 parts = 1 engine
8 million cash = 1 token
3 tokens = 2 million cash
5 engines = 1 token
Every 32 hours, the trades will refresh and go to different values (e.x. 8M cash = 1 token becomes 10M cash = 1 token.
You can't do more than two trades per day.