I used to have that problem. I'd have my taxes down to Like 4-5% and still be making $10,000 a month surplus. What causes this extra cash problem is downloads for for power or water production and garbage disposal that have an unrealistically low monthly price. Also schools, libraries, hospitals, police and fire stations that cover a great range and more sim capacity at a too low of a monthly cost. So now I use the default buildings. I have to place more of them to have coverage in all areas of the city therefore the cost goes up. I use the default power, water, and garbage too so that costs more. I still have a surplus but it's with my taxes set between 8-9%.
Terraforming in mayor mode or downloading more landmarks will get "rid" of some of the cash you have available right now, but once your done with all that, you'll be right back where you are now. Best solution is to use the default buildings to make yourself place them to cover your city's zones.
