[Deathblob]

The difference is that for each 100, it will cost you one gem (or slave to cast it).

And a mage can only use as many gems (or slaves) as his current path level.

Also, if the mage is a higher level than the spell, he gets fatigue reduction.

Example 1:

Arrow Fend is A3 to cast, and 100 fatigue.

So, an A2 can blow 2 gems on the spell to cast it. Not possible if it was 200 fatigue (since he can't blow 3 gems).

An A4 can cast it with 1 gem, and end up with ~50 fatigue. If it was 200 fatigue, even if he blows 2 gems he ends up with ~66 (=200/3) fatigue.

Example 2:

Blood Rain is B3 to cast, and 300 fatigue.

If all you have is B1 mages, it is impossible to cast without a communion. So, first you need one make to blow one slave to become a communion master, then you need to blow four slaves for all of the communion slaves (one each). Blowing all those slaves will make the casters pass out, so you have to wait and hope they don't take too long waking up. Then the master needs to blow three slaves because it is 300 fatigue.

So that's a whole lot of slaves you need to blow to cast the spell, and you will definitely be very fatigued afterwards.