Sry, but this is bullfeathers. If I can finish the tech tree at 1900 cause the AI isnt receiving enough bonus to kill me with armies twice as mine, you should think about the definition of difficulty and balance.
Instead of integrating a unnecessary mechanic for a problem, that is caused by too much money (which is created by the problem, you need a lot of money to compensate the huge production cost and the fast speed of technology) you should solve simply the root of the problem.
Too much science and too much gold production. Slow down the science progress and the production cost, and you have more time to build buildings and units with normal production, (Like it was in all other civilization games). And with even more decreased money creation, you dont have the problem of 100% purchased armies cause you cant affort it.
No need for raise of purchase cost, decreased experience or any other stupid mechanic.
ONLY. TWEEK. NUMBERS. DOWN.