Most cost-efficient conjurable units?

[Corinthian]

For troop summons in CBm its hard to beat the Hidden in Sand/Snow spells. You get some 25-50 elite undead troopers with heat/cold auras + a powerful mage on average. I once saw a single castings worth of hidden in snow troops (about 30), kill two kitted Zmeys in a battle taking only a handful of casualties. The cold auras froze them unconscious in 2-3 turns and even after loosing heads they could not flee. A casting of these spells cost 35/40 in CBM and the battle did not include the mage. Two kitted Zmeys cost around 90 gems by comparison.