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.
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Then, just wait until the year ends and the budgeting screen comes up.
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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.
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Then hit 'Go With Figures' again. Close all of your menus, and then release the L button.
The result?
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$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.