In 00_defines.lua:
WAR_SCORE_PLANET_BASE = 10.0, -- Relative warscore value of a planet
WAR_SCORE_PLANET_BUILDING = 1.0, -- Relative warscore value of a planet for every building
WAR_SCORE_OCCUPATION_MUL = 1.5, -- 150% warscore gained from completely occupying the opponents
Try making a new text file called:
"99_defines_test.lua"
Or something along that line, and place this in the same folder (X:\xxx\Steam\steamapps\common\Stellaris\common\defines)
In that text file put the following lines:
NDefines.NGameplay.WAR_SCORE_PLANET_BASE = 0
NDefines.NGameplay.WAR_SCORE_PLANET_BUILDING = 0
Then save, and try playing. This
should work out. Going to try it out right now. I'll be back with an edit.
EDIT: Doesn't work. I think this manages how much warscore OCCUPYING a planet gives you, not how much a planet costs.
This does work though:
Go to X:\xxx\Steam\steamapps\common\Stellaris\common\war_demand_types
Open the file "00_war_demand_types.txt" and right below this line:
warscore_cost = {
base = 20
You should put the following:
modifier = {
factor = -10.0
parameter

lanet.owner = {
is_ai = yes
}
}
(the forum likes to turn it into a smiley for some reason but its:
parameter: and planet.owner attached together)
This makes all 'cede planet' wargoals cost only 1 warscore, and this SHOULD only work for you as the player. I'm not sure how to make this work as a mod file (or how the game handles this) so you might need to redo this after every patch until someone tells me/us how to do that as a mod.
Cheers.
EDIT_EDIT:
Maybe this is better:
modifier = {
factor = -10.0
AND = {
parameter

lanet.owner = {
is_ai = yes
}
ROOT = {
is_ai = no
}
}
}
Because this should prevent the AI from exploiting it. I think.