The one that reduce diplo cost (the 50 thing on peace negotiations) being on Influence, if I'm right.
As of 1.8, there is no idea that can reduce coring costs, correct ?
- Thread starter BaronNoir
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I believe you are referring to Unjustified Demands, not core costs. Yes that would only be under the Influence Idea Group. It is the only way to get a lower Diplo Cost for demands in a peace deal from Idea Groups. No policies exist. You can get an additional -10% from being a Despotic Monarchy and get -33% from annex deals with a Rival.
However Religious and Expansion have CB who give you diplo free conquest of provinces.
The only issue with those CBs is that when fighting larger or comparable foes, you will need to win the majority of those battles. Not always as easy as it sounds and can sometimes cause more trouble than it was worth if you can't assure your army is better or you have dumb AI vassals/allies who keep engaging in futile skirmishes. Conquest CBs are good vs enemies who are comparable since you only need to keep taking provinces to rack up the points. You will also always have to pay for release nations and comparable options that are not direct annexation. To each his own strategy, but good luck and have fun. 
Religious with quantity and quality and maybe reformed with DoF can have some really high moral which should win you many battles.
Expansion CB you can only use aganist enemies with low tech so you should win in most chases.
Expansion CB you can only use aganist enemies with low tech so you should win in most chases.