What does the Clan Invasion casus belli cost?

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

Casus belli · Invasions

Clan Invasion

Clan Invasion is a casus belli in Crusader Kings III (invasions). If the attacker wins: The attacker vassalizes all titles within the defender's realm belonging De Jure to the target title. If the defender wins: The defender keeps their titles. Declaring it costs piety and prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/00_invasion_war.txt.

Quick answers (3)

What do you get for winning a Clan Invasion war in CK3?: The attacker vassalizes all titles within the defender's realm belonging De Jure to the target title (patch 1.20.0.3).
What happens on a white peace?: The defender keeps their titles.
What does the Clan Invasion casus belli cost?: piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The attacker vassalizes all titles within the defender's realm belonging De Jure to the target title.

White peace

The defender keeps their titles.

Defender wins

The defender keeps their titles.

If the war becomes invalid

The … has ended: the defender no longer … any land in the target region.

Who can declare it

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

Every invasions war (the group's allowed_for_character) (7 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
NOT = {
	government_has_flag = government_is_theocracy
}

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

Attacker (allowed_for_character) (30 lines of game script)

government_has_flag = government_is_clan
NAND = {
	house ?= {
		has_house_unity_parameter = unity_disable_invasions
	}
}
trigger_if = {
	limit = {
		house ?= {
			has_house_unity_parameter = unity_multiple_invasions
		}
	}
	NOT = {
		has_character_flag = unity_invasion_cooldown
	}
}
trigger_else_if = {
	limit = {
		house ?= {
			has_house_unity_parameter = unity_unlimited_invasions
		}
	}
	always = yes
}
trigger_else = {
	has_none_of_variables = {
		name = used_lifetime_invasion
		name = pending_lifetime_invasion
	}
}

As written in game/common/casus_belli_types/00_invasion_war.txt.

Defender (allowed_against_character) (6 lines of game script)

scope:attacker = {
	ALL_FALSE = {
		top_liege = scope:defender.top_liege
		liege = scope:defender
	}
}

As written in game/common/casus_belli_types/00_invasion_war.txt.

When it can start (valid_to_start) (25 lines of game script)

scope:target = {
	tier = tier_kingdom
}
scope:target = {
	trigger_if = {
		limit = {
			scope:attacker = {
				is_ai = yes
			}
		}
		any_in_de_jure_hierarchy = {
			continue = {
				tier > tier_county
			}
			count >= 5
			tier = tier_county
			holder ?= {
				OR = {
					this = scope:defender
					is_vassal_or_below_of = scope:defender
				}
			}
		}
	}
}

As written in game/common/casus_belli_types/00_invasion_war.txt.

Cost to declare

Costs piety and 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 (17 lines of game script)

piety = {
	value = 0
	add = common_cb_impious_piety_cost
}
prestige = {
	add = {
		value = 200
		desc = CB_BASE_COST
	}
	add = {
		value = 20
		multiply = scope:attacker.realm_size
		desc = CB_REALM_SIZE_COST
		max = 5000
	}
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/00_invasion_war.txt.

Source: game/common/casus_belli_types/00_invasion_war.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.