From stratics:

The Mana Cost of each special move can be reduced if the warrior's skills are high enough. Add up the skill points for Swords, Mace Fighting, Fencing, Archery, Parrying, Lumberjacking, Stealth, Poisoning, Bushido and Ninjitsu. If the total lies between 200 and 299, subtract 5 from the Mana Cost. If the total is 300 or more, subtract 10 from the Mana Cost.

Some items have a property called "reduced mana cost". These items also reduce the Mana Cost of these Special Moves.
If a special move is attempted performed within 3 seconds after another special move, the mana cost of that move will be doubled. (Never knew about this)

I have over 300 combat skill points and if I have max lmc, my ns will only cost 12 mana?? And if I cast ns repeatingly, it goes up to 24?? Can someone clarify?

Yes the mana cost is doubled if special moves are done within three seconds. Most peeps just wait the extra second before arming the special again

Vepl

I guess you dont really have to wait if you fighting peeps with evade

It's true. That's why it's hard for my Dell that only has 35 mana ^^