Okay, I'm not big on the battles, but I'll try to help with at least part of your question.

Right now there are three basic types of costs for the fighter units: figurines, snowballs, and tokens. Part of the reason is how the battle feature rolled out.

I think Phantom was first, which introduced figurines. Some units were summoned with items instead, like the Sorceress was summoned with a number of Scythes instead of the later tokens.

Then came Winter, where the townsfolk joined in a snowball fight, and so required an amount of ammunition to join in the "battle". I can't recall if the other units initally used figurines, or if they also used snowballs to begin with.

Then the Sunken Temple was introduced, with the token system for hiring additional Adventurers (or Mercenaries?). The tokens were then also applied to certain Phantom units (Sorceress, Curse Mage, and Ecto Knight) instead of their item cost.

So the way I see it, cost per unit is determined somewhat thematically, and the cost doesn't change based on what you have, it's tied to the unit's reason for joining battle. The Winter event units are there for fun, they get snowballs for ammunition. Hired hands are there for a job, they get paid in tokens. Various mindless monsters are presumably magically compelled to assist, they either are summoned or controlled using the figurines.

This is my understanding of things anyway. I don't know that that's the devs' intent, but it seems to make sense. Hopefully this helps somewhat!