All units in the game (except for most summoned units) cost upkeep each turn equal to their gold cost divided by 15. Sacred units cost half as much upkeep (gold cost divided by 30). This is not affected by Chaos Recruitment or any other rebates.
Upkeep is paid after one gets their income, and is not factored in with it.
Units that are prophets, your Pretender God, and Mercenaries never cost upkeep, much like most summonable units. Clicking on a units morale in the stat screen should show you how much they are costing you to maintain. Notably, Most troll summons (such as Sea King's Court et al), LA R'yleh's free spawn and MA Nazca's Mummies are all things that cost no gold initially (being summoned in most cases) but have an upkeep. Province Defence will never cost upkeep.
When one can't pay the upkeep, troops will start to desert and leave if their upkeep is not paid. Typically, "older" units recruited from earlier turns are prioritized for desertion, and then the most expensive units.