I think it increases over time because your income should also be increasing overtime, thus the two should balance out. However I personally don't like this system. For one it seems to me like this isn't a gradual increase, but instead it increases in incriments. Which could lead to a sudden drop of 40 GPt when unit maintainence suddenly increases. I really liked the civ3 system, but that relied on having government types which are gone.
(If you are interested) In Civ 3 each city provided an allowance of military units at no cost based on Government type and city size. For example Communism, if I remeber correctly, provided 6 units for each of the 3 sizes ( 1-5, 6-11, 12+ I think where the population sizes, town, city, metroplolis) so each city under communism allowed you to have 6 units at no cost. Having 4 cities would mean you could have 24 units at no cost, and after that each unit cost X amount of gold, aslo determined by government type. So if you had 30 units and 4 cities under communism, and unit upkeep was 1 gold per unit after 24, you would pay 6 gold per turn as your unit maintainence.