hey there - i would suggest that each city has a panel or a popup text to show its income/expense - so you can see each city quickly

for instance the panel could be out in front of the city icons when you expan them showing each item it contributes to the cause.

the popup could be every round the game goes - each city has a floating text (like the mana/health regen in tactical pop up briefly - this would be good so you could notice with out to much clutter that a city was not producing very well compared to the other cities - and you would move to investigate it.