What do you get for winning a Subjugation war in CK3?

The defender becomes the attacker's Vassal (patch 1.20.0.3).

What does the Subjugation casus belli cost?

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

Casus belli · Subjugation

Subjugation

Subjugation is a casus belli in Crusader Kings III (subjugation). If the attacker wins: The defender becomes the attacker's Vassal. If the defender wins: The defender keeps all their titles and remains independent. Declaring it costs prestige and piety. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/00_subjugation.txt.

Quick answers (3)

What do you get for winning a Subjugation war in CK3?: The defender becomes the attacker's Vassal (patch 1.20.0.3).
What happens on a white peace?: The defender keeps all their titles and remains independent.
What does the Subjugation casus belli cost?: prestige and piety; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The defender becomes the attacker's Vassal.

White peace

The defender keeps all their titles and remains independent.

Defender wins

The defender keeps all their titles and remains independent.

If the war becomes invalid

The … has ended: the defender can no longer be subjugated.

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) (30 lines of game script)

OR = {
	government_has_flag = government_is_tribal
	has_variable = koh_subjugation_cb_allowed
	AND = {
		OR = {
			faith.religious_head = this
			pam_is_antipope_trigger = yes
		}
		faith = {
			has_doctrine = doctrine_temporal_head
		}
	}
	faith = {
		has_doctrine = eastern_hostility_doctrine
	}
	house ?= {
		has_variable = subjugations_expanded
	}
}
NOR = {
	government_has_flag = government_is_landless_adventurer
	AND = {
		government_has_mechanic = administrative
		NOT = {
			house ?= {
				has_variable = subjugations_expanded
			}
		}
	}
}

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

Defender (allowed_against_character) (66 lines of game script)

scope:attacker = {
	OR = {
		AND = {
			has_variable = koh_subjugation_cb_allowed
			faith = scope:defender.faith
		}
		AND = {
			government_has_flag = government_is_tribal
			scope:defender = {
				government_has_flag = government_is_tribal
			}
		}
		AND = {
			faith = {
				has_doctrine = doctrine_temporal_head
			}
			OR = {
				AND = {
					faith.religious_head = scope:attacker
					scope:defender.faith.religious_head ?= scope:attacker.faith.religious_head
				}
				AND = {
					pam_is_antipope_trigger = yes
					rite = scope:defender.rite
				}
			}
		}
		AND = {
			faith = {
				has_doctrine = eastern_hostility_doctrine
			}
			scope:defender.faith = {
				has_doctrine = eastern_hostility_doctrine
			}
		}
		AND = {
			faith.religion = religion:islam_religion
			house ?= {
				house_head = scope:attacker
				has_variable = subjugations_expanded
			}
			scope:defender.faith.religion = religion:islam_religion
		}
	}
}
scope:defender = {
	top_liege = this
	trigger_if = {
		limit = {
			scope:attacker = {
				is_ai = no
			}
		}
		scope:attacker = {
			any_neighboring_top_liege_realm_owner = {
				this = scope:defender
			}
		}
	}
	NOR = {
		any_held_title = {
			is_holy_order = yes
		}
		faith.religious_head ?= root
	}
}

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

Cost to declare

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

Cost (30 lines of game script)

prestige = tribal_subjugation_cb_prestige_cost
piety = {
	value = 0
	if = {
		limit = {
			scope:attacker.faith.religious_head = scope:attacker
			scope:defender.faith = scope:attacker.faith
		}
		add = {
			value = 1000
			desc = CB_BASE_COST
		}
	}
	add = common_cb_impious_piety_cost
	multiply = common_cb_piety_cost_multiplier
	if = {
		limit = {
			scope:attacker = {
				has_title = title:d_sunni
				struggle:persian_struggle ?= {
					has_struggle_phase_parameter = fp3_caliph_gets_cheaper_subjugation_cb
				}
			}
		}
		multiply = {
			value = fp3_caliph_gets_cheaper_subjugation_cb_actual_value
			desc = CB_STRUGGLE_PHASE_DISCOUNT
		}
	}
}

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

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