Information Warfare is a late-game game-breaker, because it is very effective and has a constant cost. That cost is very high during the early and middle game, but late-game it is trivial. It might be worth nerfing the effects or raising the cost, but neither will completely solve the late-game problem.

Another option would be to make the cost proportional to the population of the planet. That would adjust the cost automatically, since many colonies in the late game will have large populations (even newer colonies grow pretty fast in the later game). It also seems more realistic: even with the efficiency of mass media in today's society, reaching more people means spending more money.

A third alternative, related to the both, would be to allow a sliding expenditure. That might be more difficult to implement in the current system, but I gather invasions may be redone anyway. The cost would depend on the percentage of the population you want to reach and the total population of the world (more people, more money to reach the same percentage). You could have an inverse geometric relationship, where more money adds a progressively smaller percentage of additional population reached (as you have to pursue more and more exclusive niche markets for advertisement).