What does the Eradicate Rival House casus belli cost?

prestige; the amount depends on the titles, claims and distance, worked out when you declare.

Casus belli · Subjugation

Eradicate Rival House

Eradicate Rival House is a casus belli in Crusader Kings III (subjugation). If the attacker wins: The defender's House will be eradicated by the attacker's hand! If the defender wins: The attacker's House will be eradicated by the defender. Declaring it costs prestige. The war goal is worth 1% war score. Patch 1.20.0.3, from game/common/casus_belli_types/10_tgp_japan_wars.txt.

Quick answers (3)

What do you get for winning a Eradicate Rival House war in CK3?: The defender's House will be eradicated by the attacker's hand! (patch 1.20.0.3).
What happens on a white peace?: The defender will preserve their honor.
What does the Eradicate Rival House casus belli cost?: prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The defender's House will be eradicated by the attacker's hand!

White peace

The defender will preserve their honor.

Defender wins

The attacker's House will be eradicated by the defender.

If the war becomes invalid

Eradicate House … was invalidated

Who can declare it

The game shows no text of its own for these conditions; the script is below.

Every subjugation war (the group's allowed_for_character) (13 lines of game script)

herders_and_tributary_constraints = yes
pam_personal_pacifism_offensive_war_ban_trigger = yes
tgp_dynastic_cycle_offensive_wars_ban_trigger = yes
tgp_war_ban_all_ministers_trigger = yes
OR = {
	NOT = {
		government_has_flag = government_is_theocracy
	}
	AND = {
		government_has_flag = government_is_theocracy
		has_variable = koh_subjugation_cb_allowed
	}
}

As written in game/common/casus_belli_groups/00_casus_belli_groups.txt.

Attacker (allowed_for_character) (21 lines of game script)

trigger_if = {
	limit = {
		is_ai = yes
	}
	house ?= {
		any_house_relation = {
			has_house_relation_level = feud
		}
	}
}
trigger_else = {
	house ?= {
		any_house_relation = {
			OR = {
				has_house_relation_level = quarrel
				has_house_relation_level = rivalry
				has_house_relation_level = feud
			}
		}
	}
}

As written in game/common/casus_belli_types/10_tgp_japan_wars.txt.

Defender (allowed_against_character) (5 lines of game script)

is_landed = yes
NOR = {
	is_vassal_or_below_of = scope:attacker
	is_liege_or_above_of = scope:attacker
}

As written in game/common/casus_belli_types/10_tgp_japan_wars.txt.

When it can start (valid_to_start) (5 lines of game script)

tgp_has_house_relation_level_trigger = {
	HOUSE_1_MEMBER = scope:attacker
	HOUSE_2_MEMBER = scope:defender
	LEVEL = feud
}

As written in game/common/casus_belli_types/10_tgp_japan_wars.txt.

Cost to declare

Costs prestige. The amount is worked out when you declare (by the titles targeted, claims, distance and more), so the script is shown as written:

Cost (7 lines of game script)

prestige = {
	add = {
		value = 100
		desc = CB_BASE_COST
	}
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/10_tgp_japan_wars.txt.

Source: game/common/casus_belli_types/10_tgp_japan_wars.txt and the English localisation, Crusader Kings III 1.20.0.3. “The attacker”, “the defender” stand where the game names the character; “…” marks other words it fills in during play.