spell costs:
Point system : As simple as mp, or it takes hp, or something else
- almost any RPG system
Fatigue system : Mana is tiring, just like lifting heavy rocks
- L.E. Modesitt Jr.'s Recluse books
Component system : the materials costs limit you. Can't cast 'heal' without using up a hematite...
- The Ultima games
Ritual System : Spells take time to cast.
- David Edding's version of Demonology works like this
Recharge : Each spell can only be cast so often. A spell can only be cast so often.
- Traditional AD&D magery (4th ed works differently, though)
Free : Given how easy it is to kill someone with a thrown coin, why charge a magician anything for a force bolt?
- Basically, David Edding's sorcery.
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There are more potential costs to add to this list.
* Time consumption. Not only ritual magic but all magic that has an assembly or ritual-ish component falls under this.
- Rituals
- Invocations
- Song and/or dance magic (quite akin to ritual, really)
- Runes
* Sanity. Magic is managing powers of the Beyond, and you use it on peril of your own mind.
- The Swedish RPG system "Kult" (considered one of the better horror RPG systems) and the Chtulu used this system. H.P. Lovecraft's world in its entirety is a good example of this, and most R.E. Howard's mages seems quite deranged from using their powers.
* Life. You use your own (or steal others') life essence to cast magic, shortening your life (non-replenishable)
- C.S. Friedmans' Magister Trilogy illustrates this.
* Power of magic itself. Using magic consumes the potency of magic, for the caster or for the world.
- Tolkien's magic is of this kind, with Gandalf being the prime specimen. (If you don't believe me read more than just the Rings books).
* Stability of creation. Magic is shaping chaos. Using it brings chaos into order (the creation). Disasters may occur, devilish monsters may appear.
- Old Egyptians, Babylonians and Assyrians viewed magic partly like this.
Of course, many of these are combinable. Time, sanity, life force, and stability of creation would be an excellent combination both limiting the prevalence of magic while increasing its potency and its consequences.