That is a very interesting and good idea. And in theory, not too difficult to implement I would hope.
Great suggestion, I hope they add that soon. Perhaps a cooldown timer as well, so that if a truce is made by nonpayment or expiration of a war, one cannot simply constantly start it up again without a brief period of peace... unless they wanted to further add to the alignment system and have a high negative alignment to killing outside of war or declaring war too soon after a peace?
But I vote yes! cost of war is a great idea