how is unit-upkeep calculated?

What is the difference between resources and gold? Gold is generated by every province, and stored in your treasury. It is used to buy troops, hire mercenaries, and build buildings. Gold is also used to support troops, normally at a rate of (initial cost)/15 per turn. Resources, on the other hand, cannot be stored - the represent the amount of labor or production capability of a province. Heavy armors and weapons require more resources than, say, leather armors, spears, and slings.

Though Cherry might want to just make it simple.

Upkeep is split into two parts. Sacred and Non-Sacred.

Non-Sacred Upkeep = (Initial Cost)/15 Per Turn
Sacred Upkeep = (Initial Cost/30 Per Turn

A number of Summons (like Trolls) have an initial cost variable while others do not that is not usually visible. Only a few types of summons are upkeep free.