Excellent idea for a game.
Here's my strategy for making money at the beginning fast:
1. Cover your entire area with lv1 cities.
2. Estimate how many cities you want at the end.
3. Commence upgrading systematically each city you want to keep (don't upgrade a single one, upgrade them in order so you can save money).
4. Continue doing this until you run out of turns.
5. Wait for turns to roll in.
6. Repeat steps 3-5 until you can't stand it any longer
7. Profit.
For the benefit of Newgrounds!