I am not sure that the AI is getting extra cash. It never did in STW or MTW. I would think that the reason the AI builds cash quickly is that they don't spend much of their money the first few years. That is why it is so easy to eliminate the Scipii quickly because they don't commit the funds to build military the same way we do.

It appears that the AI is using the same principles that the rankings are based on to make decisions. If savings were not calculated into the overall rankings the AI might do a substantially better job of providing a challenge. I wonder if this is moddable?