HINIRANG the RPG: Update
I've got the broad strokes down for the magic system creation system! What? Oh, it's a system that allows you to create magical systems in the same game... this goes for item creation and a number of the Katao, Tsino, and Ispancialo magical systems.

Basically, there's an inherent difficulty in casting spells. Spell systems lower or increase that difficulty, but tend to trade off the flexibility or type of spells that can be cast. There's a limit to the amount of power you can "channel", and some rationale for blood sacrifices and bonuses to theurgical rites through fasting and self-denial. Also, certain sources of magical power have inherent limitations on them as well, and these impact the cost of the mana level you can channel safely.