When setting an item cost on a spell to 0, it defaults back to 1.
I have the following on a spell:

    costs:
        mana: 15
        fire_charge: "1 * nether_attune"

Intended function is that players with the "nether_attune" attribute set to 0 do not need a fire_charge to cast the spell, but it didn't work. I tried this to test it:

    costs:
        mana: 15
        fire_charge: 0

And sure enough, it still didn't work - the player still couldn't cast the spell without 1 fire charge