For basic "Upkeep" there are two varieties.

Upkeep from Normal [Gold Cost] units which is 1/15th of the initial cost of the unit (60 Gold = 4 Upkeep)

The second is Sacred Units (Those with the Sacred or "Priest Levels" symbol in their special abilties) their upkeep is 1/30th of the Initial gold cost of the unit (60 Gold = 2 Upkeep).

Summoned monsters may or may not have upkeep depending on if they have been given an initial gold cost (even though you don't pay it, you summon the monster). My guide has most of the summoned upkeeps to a degree of +-.20 accurate (that means I summoned 5 of them and if I didn't see a change in upkeep I assumed it was 0).