Oh and i have a suggestion,
What about different modes like:
Easy: 20min
Normal: 10min
Hard: 5min
Extreme: 2min
And also different money multipliers like:
0.3x
0.5x
1.0x
2.0x
3.0x
In a menu before you start the game, it would deffinetly be a big update and would require a bit of time, bit many people may enjoy getting an easier start, before they try a harder gamemode to test their capabilities.