During each update, nations must pay to maintain their units, ships and planes.
The cost to maintain a ship, or land-unit is:
.001 * cost of thing * ETUs/update
The cost to maintain a plane is:
.001 * cost of thing * ETUs/update + 5 * mil_cost * crew
Note that this cost can be changed by the deity.
Also note that if you have purchased high-tech items, and your tech is not at least 85% of the item's tech, you are charged double to maintain the item.
Also note that engineering land units costs 3 times what normal units cost, after the tech is figured in.
If you can not afford to pay maintenance costs then your unit will lose ETU/5 points of efficiency. It won't go below its minimum efficiency, though. Thus, if you were playing in a 60 ETU game, and you did not pay your maintenance costs for a 40% frigate, then the frigate would drop to 28% efficiency at the first update, and to 20% at the second update.
See also : Unit-types , Ship-types , Plane-types , Ships , LandUnits , Planes , Updates