Yeah, that system is not really set in stone.
This was said by Olof in the Video Thread. The difficulty levels are currently temporary. Just to test the game. I hope they configure it before the first release. Sandbox will definitely be necessary. (Later). Because some people like me, plan to constantly spend money millions without any plans for profit. So for us a Billion isn't enough. It'll probably get over even before I build half my airport.
Also I think it'll be easy to change values using the Cheat Engine. So I'll make my starting $1 billion into 100 or 1trillion???