[llamabeast]

Your maths is not correct at all, because the mages (should) survive the battle. So you can't calculate the cost of a shadow blast as being (cost of mage) + (cost of gem).

I suppose if you wanted to analyse it like that, you'd have to consider the expected number of castings the mage would make over his entire life. Say you'd expect N. Then the question is whether the troops killed by N shadow blasts cost more than N death gems and one death mage.

Anyway, Shadow Blast is an awesome spell, but should be saved for special occasions, when you really NEED to win. Mostly, as other people have mentioned, you should use skelly spam.