A common agreement when people talk about fictional magic is "all magic has a cost." It's sort of the basis of a lot of tropes. Like, a lot. (Though some of those tropes are more about limitations than costs, and that falls under your Path.)
You see this in a ton of systems. Some are pure mana-point-oriented (Rifts, Exalted, D&D psionics), though sometimes you have to do weird stuff to get those MP and they're limited in weird ways (Unknown Armies, standard D&D spellcasting). Others allow free use below a certain level, but cost you when you're trying to push your skills (Mage, Ars Magica, Nobilis, Mutants & Masterminds). Others have costs that are tied into other parts of the system (Call of Cthulhu, GURPS, Hero, Shadowrun). In fairy tales, magic's price is more often something bizarre or dangerous. When you look at RPGs, though, this tends to be limited to really specific characters in a handful of games (Exalted's Sidereal paradox, the Wasteland effect in Promethean, Dark Sun defilers) because it can get real wacky real fast if you're expecting to throw spells around like a D&D wizard but you need to take the equivalent of a Moderate Complication every time.
In SsA, it feels like costs could most easily be described in game terms as Complications. Certain Paths will definitely be tying into Complications a LOT - wild magic, for example. (I'm certainly not going to bolt magic points onto SA.) But before we start charging Complications for magic on the regular, what are some alternatives?
- Power-limited. You can do anything up to a certain power level and there's no pushing it.
- Skill-limites. You have the power for anything, but not the skill for it. This can easily become a "pay for overcasting" setup if the raw power screws you over when it exceeds your skill.
- There's a cosmic arbiter who accepts or refuses each spell as it's cast. (This might seem like a game of Mother May I with the GM, but remember that prayers, as described in several game systems and by religious believers in real life, basically work this way.)
- The cost might be a Plot-Driving Weakness. It doesn't necessarily cost you, but it makes it easier for someone to exact a cost from you. Think of Morpheus (from Sandman) losing his tools before the story begins and needing to recover them later, or a lich hiding their phylactery because if someone gets their hands on it they're no longer immortal. Or Sauron's ring.
I certainly don't want to have characters pay for every damn time they use magic. That doesn't fit the feel of the game. I'm likely to have some mixture of things, with most actions not requiring a cost but things on the Infrastructure level or that push your own Capabilities invoking some sort of Complication. However, I'm not even sure that a transactional view of magic in general is appropriate to SsA, and for certain Aum will have paid many costs to begin with. Still considering.
Side note: magic in books and fairy tales is done without price all the time by characters that would be classified as NPCs in most games. The transactional view of magic is only reliably applied to main characters. We'll be doing away with that conceit, thank you very much.