[sector24]

Skellispam is definitely the most cost efficient move for C'tis. You're going to get much more than 5 casts of Raise Dead per Sauromancer because as you flood the battlefield with skeletons, your mages have time for their fatigue to dip below 100 and cast it again. Also, your opponent's fatigue starts to build and as the battle drags on they start taking more hits.

Shadow Blast is much more useful for removing difficult units from the battlefield than it is for just general destruction. For instance fighting double blessed troops that could kill 10+ Elite Warriors before going down would be a good time to use Shadow Blast. But the computer obviously doesn't do that so I don't think there's ever really a need for that spell against the AI.

To take your argument to extremes, it's much more cost effective to put 125 PD into every province than it is to recruit Elite Warriors. For a one time investment of 8,000 gold your province becomes impregnable for the rest of the game. The AI almost never builds the right troops in the right numbers to defeat a free army like that.

The problem with Elite Warriors and PD is that they aren't time efficient. It's going to take years and years to conquer the enemy a few provinces at a time. With magic, once you get the spells you need you can teleport in raiders, wipe out huge armies with just a couple spells, basically redraw the map in a couple turns. It's not cost effective for a long time, but it is efficient.

PD: Cost Effective, not efficient
Elite Warriors: Somewhat cost effective, somewhat efficient
Mages: Not cost effective, very efficient (becomes moreso at higher research levels)