no one has mentioned in this thread yet:
the ability for spies to destroy a random improvement.
i think ideally you could break it down to two groups:
*destroy 1 random income improvement (banks, schools, etc): $500,000 + (5 x enemy nation strength)
*destroy 1 random military improvement (barracks, guerilla camps, etc): $300,000 + (2 x enemy nation strength)
you may say "well what's the point? improvements are cheap," but if the nation you performed these on were saving up for a few days of collection, or you were about to attack them with a HUGE deployement, it could really be worth it.
prices are just suggestions. and ideally, the improvement could not be rebuilt in a day or two.