Nonsense.
Metal is a resource for war. I can crack up taxes for one season, and pocket 200 Gildar. But when I need metal to build one of my heavy cavalries, there's no easy way to pull what I need out of my ass. Not unless you make metal really cheap in diplomacy.
And by the way, you cannot get 6 Gildar for metal unless the AI has ran out. Talk to an AI with 300 loads of metal in its stores, and it will offer you a grand total of 0. I'd say that it's one of the BEST parts of Fallen Enchantress's diplomacy.
If anything, Crystal and metal are so hard to get nowadays (In the quantities needed for troops) that the AI is UNDERvaluing them in diplomacy. I need 110 crystal for one unit of my Elusive Lancers in my current play through . I have been getting them by killing Ophidians and trading with the AI. Do you know how long it would take me to actually mine 110 crystal? No, you do not. Hell, I wish I did not.
And stop trying to balance everything in the sense that "everything is balanced if everything work the same, namely not too well"