Yeah so in Fate of Cthulu, doing magic can have a cost because you are playing with Great Old Ones' technology. Everytime you use magic, you tick a clock to track how corrupted you get over time (so kind of like the opposite of a mana clock I guess?
When the clock is filled, you character's marks one of their aspect as "corrupted"
Anyway, perhaps it's an interesting example to highlight in the sense of: magic can consume your reserves, but the cost of using magic could also be how corrupted you get from using it.
Or perhaps that's out of scope and this should be its own extra!