I'm not expert about magic (I have always been the warrior, in fact), but there are many spells which cost over 100 in magicka and I am already at the maximum level of intelligence (but apparently not in real life asdasd XD ). I know there are potions and spells of Fortify Magicka or Fortify Intelligence, but they give max 20 points and it's not enough for spell that cost 200 or more.

So, how can I cast high cost spells?

You can make potions and spells that do way more than fortify for 20 points. Max out your alchemy. IIRC, the potions stack too.

Also, if you have the fortify skill spell, then a higher skill level will make the spells cheaper to cast.

Will it? I thought mods (like Cast Reduce) did that, not the vanilla game.

What do you mean? Every spell has its relative cost and it seems to be constant. For example the spell "Panacea", as you can see here: http://www.uesp.net/wiki/Morrowind:Spells/Panacea#Panacea

costs 120 magicka and it's the same cost that I see in the game too. It's not cheaper (and I raised Restoration).

For the potions ok, I'll try

I am not sure, but I think that when there are ranges for a spell effect, such as 5-10, it scales with something (I am not sure with what-level, stat, or ability). I'm pretty sure that my Absorb Heath (which costs 11 mana) and absorbs for 5-52 on touch, it definately does more than 5 on a regular basis, and I think for me it does around 30 points. My mysticism is around 45, my level is around 18, and my willpower is 44.

your absorb health it's a base spell or a created spell? And you play with mods or the vanilla game?

In addition to fortifying your abilities, starting with a birthsign of the Mage gives you an additional magicka at .5 times your intelligence while the Apprentice gives you an additional 1.5. The Atronach gives you double your intelligence. All of them have their downsides, however. You can find them http://www.uesp.net/wiki/Morrowind:Birthsigns.

As well, the Morrowind Code Patch adds a couple of items at the start that fortify your magicka, as well as a few other skills and stats.

thanks, but I play with the vanilla version of the game and I don't want restart the game :\ I think the only way are the potions.

It is a base spell, and I was playing with a completely vanilla game. No chocolate involved. However, I have since installed Tribunal, and so cannot test it within the same environment.

that's not a problem, I play the Goty version (so with Tribunal and Bloodmoon) but also in the internet guide, the spell costs are marked as invariable. With the school magic level change the chance to cast a spell, but not the cost. Or at least it's so in my game and in the UESP there is written nowhere the opposite.

I'm not sure, but another thing is that a higher maximum magnitude might (If I recall correctly) lower the chance faster than it raises the cost. So put in a higher magnitude and you get more powerful spells.

Well, if magic is not your specialty, why not enchant a few items with the desired effects and make your local enchanter happy/rich?