I'm sorry but I'm going to have to disagree. Imho making t5 troop prices exorbitant is the wrong way to go. It means there's no reason to ever have t5 troops and they might as well not exist. What you're talking about is penalizing the player because the ai is . If they wanted to make leveling up to tier 5 troops a much harder process I'd be fine with that and I also feel that t6 troops should be rare so no one, player or ai, can run around with an army full of KG or Fian Champions. But if the ai is so bad that they keep throwing low tier troops against me that's not my fault that's something that Taleworlds needs address without unduly penalizing players. If the ai loses a big battle or 2 they need to turtle up and start training then come back or sue for peace, not keep recruiting a bunch of low level troops to throw at me again and again because that's not a close approximation of how it worked and more importantly it makes for boring gaming which is what we have now. The ai battle mechanic in this game is bad so what you're asking is to fix the problem by essentially making top troops useless. That's not going to sit well with a lot of players.Personally I think that Tier 4 troops should be the optimal "bang for the buck" for the player, with T5 being less efficient, and T6 even less so.
In other words, there should be diminishing returns when you upgrade your troops beyond T3-T4, where the increase in cost is bigger than the increase in lethality. Just how big this increase would be is of course subject to balancing, so I'm not going to give any particular numbers.
I also believe that all of this should be focused on the player and not affect the AI, as AI armies are already plagued by being 70% recruits after getting beaten once.
I know that TW tries to make it so that the AI and the player play by (roughly) the same rules, but I think that outside of eliminating blatant AI cheating, this is not actually a sustainable strategy. The player is way too good at using their resources compared to the AI, so the bots really do need handicaps, it's all about making them not so obvious and jarring.