I'll paste a page from my Dark Fantasy Magic. Hope it's useful.
https://www.drivethrurpg.com/product/284302/Dark-Fantasy-MagicThe cost of magic
The cost of magic is one of the most important aspect of any magic system, and it is particularly important in dark fantasy games, where magic is often dangerous, corrupting or downright evil. If magic doesn’t have a cost, it becomes simply a supernatural skill or superpower. Anyone with a modicum of talent will become a wizard, and magic becomes commonplace and indistinguishable from technology, with magic missiles replacing artillery and portals replacing airplanes.
In game terms, magic without a cost (other than training, etc.) creates other issues. If anyone that is sufficiently smart can become a wizard, all the best scribes, doctors, tacticians and mathematicians will be wizards. If you use charisma instead of intelligence, for example, there will be no swindlers or mundane leaders – anyone with that kind of charisma would easily become a real magician! A world so full of wizards might be very interesting, but it is not often found in the dark fantasy works that inspire this book.
The price of magic cannot be simple gold, either, or it becomes a commodity (although that might be interesting on its own). It must be something significant – maybe it requires complete devotion, madness, human sacrifice, great risks to the wizard’s body or soul, or maybe it poses an existential risk to the world itself. The options are many… fortunately, we have some random tables for that! Of course, you might prefer tamer options – maybe the wizard get exhausted or temporally forgets a spell. We have tables for that too.
Costs, great and small
Here are some possibilities about the cost of casting a spell.
No cost. You can cast spells “for free”. It becomes just a useful tool, or a bow with infinite arrows. While it is reasonable to think that a powerful sorcerer would be able to cast simple spells with impunity, this does not fit the dark fantasy tone we are looking for. Use it with great care, or magic will become mundane quick.
Small cost. Casting any spell has a small cost. Maybe it is spell slots, a few hit points, some gold or material components, or just a small penalty you get when casting more spells in the same day. The wizard is fine after casting a spell and, even if she cannot cast it repeatedly, she can probably do it again tomorrow.
Moderate cost. You need to spend a significant time of wealth, time or energy to cast a spell. It will affect not only your ability to perform cast more spells, but may temporarily affect your health, memory, etc., hurt your friends, or ruin the environment.
Severe cost. The cost to cast a spell is great – maybe too great. Human sacrifice is not uncommon amongst evil wizards, but human life is cheap in dark fantasy, and magic can be much more expensive. Maybe this spell requires the destruction of a rare gem or magic item; the blood of a king or the bones of a dragon; the life of a loved one; the permanent maiming of the wizard’s body, mind or soul.
In general, the cost of magic will be somewhat analogous to the effects of a spell, i.e., an impressive spell will have a significant cost. Role-playing games traditionally use “spell levels” to measure the potency of a spell; when spell levels are used, the cost should be adequate to spell level, as seem below.
Notice that some costs (especially severe costs) may be relative to the magic user. For example, destroying a loved possession or a loved one is always more powerful than sacrificing a stranger. Giving away pieces of your soul becomes diminishingly effective as it becomes corrupted. And so on.
Needless to say, the cost of magic should not be easily circumvented. If spells cost hit points, you cannot replenish these hit point by using another spell!
Risk & randomness
In addition to the expected costs, magic creates risks. The most obvious risk is that spells can go wrong – missing the target, giving false (or dubious) information, etc. There can also be a possibility of spell mishaps. Maybe the spell blows up in your face, hits an ally, or calls the attention of evil forces. While risky magic adds mystery to the system, making things too random will generate wacky results, as discussed above. To avoid this, it is useful to make the risks either fixed (for example, a failed spell will always cause exhaustion, or turn against the caster, etc.) or reasonably well suited to that one particular spell (for example, a failed fire spell will hit an ally, but a failed portal spell will send you to the wrong place).
Small costs – slots, points, etc.
“Ordinary” uses of magic always carry a small cost. Here are some examples; there are suited to “traditional” or “old school” games, but adaptable to other systems. You are not supposed to use all of them in the same setting, or roll randomly every time. Instead, chose one (or a few) to decide how magic works in your setting.
Although these ideas deal with game mechanics, flavor is also important. For example, spell slots will give you a “Vancian” feel, while using hit points to fuel magic will bring many other associations. This loss of hit points may represent a wizard cutting himself when casting spells, a loss of vital essence, a simple expenditure of calories, etc. In addition, it will require wizards to be more resilient than common people. Chose the explanation that best suits you game.
Here are a few alternatives to spell slots.
Hit points. You can use HP to fuel your spells. It costs 1 HP per spell level, for example; a decent die roll should reduce this cost by half (or other amount).
Magic points. You have a separate pool of magic points. It works similarly to HP, but can only be used for spells. Magic points are especially interesting because you could fill this pool with sacrifices, rituals, etc. (see moderate costs and severe costs below). Using a term such as “ka”, “mana”, “essence”, or “souls” instead of the mundane “magic points” is a good idea to make magic sound less prosaic.
Alternatively, you can have a “resolve” or “luck” pool that can be used for magic AND other adequate activities, so even non-casters can use it.
Penalties. Every time you cast a spell, you get a penalty to cast the same spell in the same day (for example, -1 per spell level). This forces the wizard to rely on different spells instead of always using the same one.
Material components. Casting spell requires special ingredients. These might be expensive, heavy, consumable or hard to find. For example, if you require diamonds to cast spells, the cost rises with spell level (maybe exponentially so, if characters have a lot of money). Some ingredients are just impractical - even if the sacrifice of a single chicken is required, adventuring magicians will have a hard time traversing dungeons while carrying lots of live birds in their backpacks!
Moderate costs – pain and destruction
Here is a list of catastrophes that magic can cause. I use it for spell “fumbles”, which means a bad die roll when attempting to cast a difficult spell. You can rule, instead, that every spell has a small chance (maybe 5% per spell level) to cause these results – even if the spell is successful. Once again, this shouldn’t be easily circumvented – if you get a result that would harm you allies but you have no allies nearby, a distant loved one may get hurt.
The effects that affect the spellcaster directly are not permanent, but should last for at least a day.
d20 Cost
1 Desolation. Flora and small fauna wither and die around the spellcaster.
2 Destruction. The spellcaster’s allies or friends suffer lose a total of 4 hit points per spell level.
3 Retribution. The spell turns against the caster. No save is possible.
4 Confusion. The spell gets out of control and affects another random target.
5 Inversion. The spell’s effect is the opposite of the intended.
6 Oblivion. The spellcaster temporarily loses one point of Intelligence per spell level.
7 Exhaustion. The spellcaster temporarily loses one point of Constitution per spell level.
8 Exsanguination. The spellcaster loses 4 hit points per spell level.
9 Mutation. The spellcaster’s body becomes warped (check the “marks of corruption” table).
10 Provocation. The spellcaster draws hostility from animals, monsters and unfriendly people.
11 Suspicion. The spellcaster becomes suspicious to everyone, even when silent.
12 Activation. Hybernating creatures awake, the half-dead rise, old spells become active.
13 Demolition. Structures fail and objects fall around the spellcaster.
14 Deprivation. The spellcaster’s clothes and mundane items become useless.
15 Ruination. One magic item from the spellcaster gets destroyed.
16 Putrefaction. Food and water all around are ruined.
17 Distraction. The spellcaster is unable to cast spells for the day.
18 Stigmatization. The spellcaster gains a terrible mark on the face.
19 Convocation. A demon or other supernatural entity is summoned or takes interest on the spellcaster.
20 Debilitation. The spellcaster becomes harder to heal, resurrect, etc. (-2 to the healer per spell level).
Severe costs - sacrifice
Severe costs make powerful rituals possible. Earth-shattering spells, magic items, invocation of elder demons, permanent changes to the world… The sky is the limit (or, maybe, the Abyss). Here are some terrible costs (or severe limitations) to fuel your spells. It often requires the sacrifice of something incredibly valuable.
d10 Cost
1 Human sacrifice. Self-explanatory, but see “Costs, great and small” (above).
2 Animal sacrifice. Only powerful animals or nearly extinct species.
3 Mutilation. The spellcaster must give up a finger, eye, etc., permanently.
4 Destruction. The spell requires the destruction of an artifact or magic item.
5 The stars are right. The spell can only be cast in rare circumstances.
6 Deicide. You need a deities’ blood or parts to cast the spell.
7 Time. Casting this spell requires a lot of time of work and isolation – maybe years.
8 Cataclysm. The entire place (castle, village, valley, etc.) becomes wasted and damned for centuries.
9 Damnation. Casting this spell requires a willing soul to be given away.
10 Impotence. This is the last spell the wizard will ever cast.