Army upkeep.

[Ludens]

When a city produces a unit of soldiers is that city then continually charged for that particular unit's upkeep or are the taxes decided differently?

It's done differently: the total upkeep of units, ships, agents and family members is divided across all cities of your empire, with the larger cities paying more. Because tax/trade income does not grow proportionally with population, this often leads to the largest cities in your empire (which produce the most income) seemingly being at a deficit. This is because they bear a disproportionate amount of unit upkeep - in reality, they contribute most to your economy. You can see how much an individual city pays for upkeep on the detail scroll.