I'm surprised you actually upgraded the Finance Center. You gain a very small increase in your interest for 1600 cash. Not really worth it. I usually build the finance center for the minor benefit it gives, since it only costs 100 cash to build, but I never do anything with it after that.

I suggest attempting one of the "hard" levels. I've managed to beat one of them, but they're a lot harder due to the fact that you start with less money and there's less "walkway" area.