Mmmm I dunno what to say about this: the simplest change would be just increasing the amount of money you earn per second, for example from 25$ to 100$. Implementing a proportional formula is a two sided weapon: if the money you earn is, as you suggest, proportional to what you already have, it's true that the richest players will go there to earn more money and then other players can kill them, but as Hazard said you are making the rich even richer, unbalancing the game even more. If by the contrary, you earn more money when you are poor and less when you are rich, then you have a more balanced game but the richest players will never go to the ship...
So +0