How is upkeep cost determined for a unit?

[rogerbacon]

OK, I figured out that upkeep cost is a calculated field but I have no idea which components go into determining that value. Does anyone know?

[georgios]

Most possible the GDP/c value or inflation.

[barkhauer]

Base maint. cost, inflation, and maintenance/training slider in defense tab (ie, the one other than salaries.) You can primarily lower this cost by reducing the slider, but at the cost of lower efficiency.