Huh, war costs nothing? You are taking men away from farming fields, production, etc. Therefore you are losing revenue from said activities.
You must provide weapons, armor, training, travel, food, water for weeks on end. You must have well paid people to organize said thing. In those times, and especially in our times. War is extremely expensive.
Depending on the culture.
Rus, Danes, Norse, and Swedes, it would cost very little... as each man and woman was required to keep weapons and armor available for call to arms by their Jarl.
The Japanese, Mongolians and Chinese all kept a standing military.
The Syrians, Ottoman, and Al-Andulus empires ALL kept standing armies of professional soldiers.
So really only Europe mainland and England had issues with cost of raising an army.
Just to give you an estimate the battle of Hastings, in 1066, was far less costly to William, to execute than to Harold to defend. (and lose..)
Harold had to rally his men, which were all farmers, smithies and a few nobility who were trained to fight. William had a mostly professional standing military, few actual conscripted individuals. His biggest expense was crossing the channel.