SimCity can be a tough game if you're not that great at managing the budget, which is why I usually don't bother worrying about it.

First, spend all your money. Make sure you spend it on stuff like roads and police/fire departments, stuff that will want tax money to keep operating. Then, wait until the end of the year.

Then, just wait until the year ends and the budgeting screen comes up.

Start holding down the L button (and don't let it go until I tell you), and hit 'Go With Figures'

Then, go back into your Budget screen and raise all of the Funding Levels to 100%, which will result in negative Current Funds.

Then hit 'Go With Figures' again. Close all of your menus, and then release the L button.

The result?

$999,999 in the city's treasury. With that kind of cash you can do pretty much anything you want any time you want.

Go nuts.