How to make money?

You want to be spending donuts on mystery boxes. You will get Kearney, Blue Haired Lawyer, and Squeaky Voiced Teen pretty quickly and they all make a lot of money from tasks. Your multiplier for cash will go up, and you should also get Lard Lad buildings which are a quite decent source of income. Avoid event questlines and try to unlock as many characters as possible.

I wouldn't bother with the springfield downs, others may disagree, but in my experience the long term result is you get back what you bet in.