What do you get for winning a Retaliation War war in CK3?

The attacker makes the defender their tributary (patch 1.20.0.3).

What does the Retaliation War casus belli cost?

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

Casus belli · Humiliation

Retaliation War

The war is called “Retaliation War”

Retaliation War is a casus belli in Crusader Kings III (humiliation). If the attacker wins: The attacker makes the defender their tributary. If the defender wins: … does not become … tributary. Declaring it costs prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/09_mpo_wars.txt.

Quick answers (3)

What do you get for winning a Retaliation War war in CK3?: The attacker makes the defender their tributary (patch 1.20.0.3).
What happens on a white peace?: The defender does not become the attacker's tributary.
What does the Retaliation War 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 attacker makes the defender their tributary.

White peace

The defender does not become the attacker's tributary.

Defender wins

… does not become … tributary.

If the war becomes invalid

Retaliation War invalidated!

Who can declare it

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

Every humiliation war (the group's allowed_for_character) (5 lines of game script)

herders_and_tributary_constraints = yes
pam_personal_pacifism_offensive_war_ban_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) (8 lines of game script)

government_allows = obedience
has_variable_list = retaliation_cb_var_list
exists = scope:defender
is_target_in_variable_list = {
	name = retaliation_cb_var_list
	target = scope:defender
}
is_landed = yes

As written in game/common/casus_belli_types/09_mpo_wars.txt.

Defender (allowed_against_character) (3 lines of game script)

scope:defender = {
	government_has_flag = government_is_nomadic
}

As written in game/common/casus_belli_types/09_mpo_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 (26 lines of game script)

prestige = {
	add = {
		value = 75
		desc = CB_BASE_COST
	}
	multiply = common_cb_prestige_cost_multiplier
	if = {
		limit = {
			scope:attacker = {
				any_character_situation = {
					any_situation_sub_region = {
						has_sub_region_phase_parameter = the_great_steppe_free_retaliation
						any_situation_sub_region_participant_group = {
							participant_group_type = nomad_rulers_capital
							participant_group_has_character = scope:attacker
						}
					}
				}
			}
		}
		multiply = {
			value = 0
			desc = CB_STEPPE_SEASON_DISCOUNT
		}
	}
}

As written in game/common/casus_belli_types/09_mpo_wars.txt.

Source: game/common/casus_belli_types/09_mpo_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.