I think the only way to balance spies is to make it so there is no limit on how many times a nation can get spied on a day. However, nations can only launch one spy attack a day. To balance this, the chances that a spying mission will be caught should increase exponentially at each threat level. For example, at threat level 5 (lowest), the chances of catching a spy is 100%. However at threat level 1, the chances of catching a spy should be 10000% or another acceptable value at threat level 1. This way, a nation is expecting a lot of spy attacks by say an entire alliance, he has proper defenses. However, putting your threat level at 1 should have greater repercussions. Most of the time, a nation will have a threat level of 5, and have few spy attacks.