So the AI in my metagames keeps running out of money, making for easy fights. I was wondering if it's at all possible to use the same code that makes non-native techs 4x more expensive to make things the AI buys say, 0.75x the price to reduce the total cost? Or perhaps make it possible for them to get more money each turn from their sites?

Nothing so complex, check the Metagame.rte folder. And change the save of the game for desired effects. You can make your game insane by giving each of the AI like 40,000 while keeping your own at 5.000.

Otherwise, I'm unsure as to change them as a general rule.