Mmm - it makes sense that many if not most of the magical summonses wouldn't require upkeep - in fantasy monsters tend to stay in line from fear, and the promise of fresh human blood and meat, or 'intangibles' which the mage or master provides.
However - a set of spells that allowed regular units to be ensorcelled, enchanted so they no longer pined for home and family, no longer expected pay? A nature version should possibly even eliminate the need for food - mortals spent decades in elven halls from just a few nibbles of enchanted food.
Then again, nature already is the only (?) way to provide supplies via magic items. Seems like earth and blood might be other realms that could provide food ( via the earth's bounty, which might be stepping on nature's toes a bit, and blood via magic cauldrons in which bodies of the fallen are turned into food. Maybe something which lowered population of the province the army was in....
)
Still - if Dom3 could have a few ways in which upkeep could be reduced or eliminated for regular troops, that might help keep recruitable troops in play in larger quantities, longer in the game.
Nature, Blood ( a blood binding ), and Astral (which already has Enslavement ) all seem like likely options for such spells / items.