The base cost of unit(rts resource), spell/skill(mana in rpg) typically remains stable, but this mechanic changes the balance:
1.each upgrade raise an element of base cost or base time.
For RTS: unit cost and training time rise proportionally to number of upgrades. Individual upgrades do the same to unit cooldown or energy cost.
For RPG: skill/spell cost(mana/resource/element units) and cooldown rise proportionally with levels or component upgrade level.
2.Balance chosen: A.cost vs effect, higher cost means more effect. B. cooldown vs effect, higher cooldown means more effect.
1.prevents overpowered "uber-units" from dominating the game.
2.Balances cost of upgrades vs their effect: effective soft cap for upgrade, as too much upgrade cripple efficiency.