I'm getting really curious about this as well. Since I saw somewhere the mention of "Magic Points" on the character sheet, I kind of hope it isn't just a "mana pool" system. I don't like situations where you can't do your "cool move" because you're out of mana/spell slots... I'm hoping for an original mechanic using the strengths of the Year Zero system, I think mutations in MY0 did that well!
I realize it's early for this kind of thoughts, please excuse my hunger for rules sneak peeks
How about if magic cost are based on spell difficulty, (higher the spell the higher the cost) but the cost can be reduced by any successes beyond that need to make the spell?
So if you only need 1 success to cast the spell, but you rolled 3 successes, you get to reduce the cost in mana by 2 point,
(After you run out of mana, I'd make it you need to roll Strength equal to the cost or fail, with the option of temporary reducing your strength to cast spell, (so if you had a Strength of 3, you could get three extra mana, but must reduce your strength by 1, if you Strength hits zero, you die,)