You can change it in //ImperatorRome/game/common/wargoals/00_default.txt .
Just change each war's conquer_cost modifier to your liking.
The default for a normal war goal is conquer_cost = -0.25 so it's already a 25% decrease.
The superiority war goals have instead a modifier increasing the costs when taking land.
They don't stack, each war goal's cost only affects that war goal.
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Or I guess you could add a modifier to the game's base values that affect everyone.
It's in //ImperatorRome/game/common/modifiers/00_hardcoded.txt .
The base values start at line 698 or you can search for base_values .
You can add the following line there:
war_score_cost = -0.2
The above number means that war score cost is now reduced by 20%.
BUT that also affects all other war score actions such as taking vassals in war.