Most cost-efficient conjurable units?

[Immaculate]

this doesn't answer your question but there are certain summons who's efficiency can be maximized by the caster.

A few examples:

• Ghosts are summoned at a rate dependent upon the caster's death magic rating. So its low efficiency with a low death magic caster but increasingly better with better death magic.
• Lamias are summoned at a rate dependent upon the caster's nature magic and lamia queens and some pretenders get a bonus to summon these. So if you have a high nature lamia queen available, or a very high nature serpent king, then these become very high efficiency summons.
• Drakes and Wyverns are summoned at a rate of 3 per summon when the caster has 'dragon master' (enchantment 3) but only at a rate of 1 per cast otherwise so the efficiency of summoning them is tripled with the 'dragon master' spell (which costs i think 5 nature gems?)

Personally the value of summons is completely dependent upon your strategy. I don't think of any of the summons as stand-alone troops but rather consider how they are synergizing with the rest of my strategy. Ghosts by themselves are not great. Ghosts in the dark with cold-generating, cold-immune units, maybe with a wailing wind or rain of blood are suddenly much much better.

So efficiency counts but then so does use.