What does the Plunder for Contribution casus belli cost?

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

Casus belli · Mandala

Plunder for Contribution

All Under Heaven

Plunder for Contribution is a casus belli in Crusader Kings III (mandala). If the attacker wins: The defender is forced to Contribute to your Great Project. If the defender wins: The attacker's divine reputation is tarnished. Declaring it costs piety. The war goal is worth 1% war score. It needs All Under Heaven. Patch 1.20.0.3, from game/common/casus_belli_types/tgp_eastasia_wars.txt.

Quick answers (3)

What do you get for winning a Plunder for Contribution war in CK3?: The defender is forced to Contribute to your Great Project (patch 1.20.0.3).
What happens on a white peace?: The defender does not Contribute to your Great Project.
What does the Plunder for Contribution casus belli cost?: piety; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The defender is forced to Contribute to your Great Project.

White peace

The defender does not Contribute to your Great Project.

Defender wins

The attacker's divine reputation is tarnished.

If the war becomes invalid

The … has ended: either the attacker … no longer an Aspect of Destruction Mandala Ruler with a planned Great Project, or the defender … no longer a Mandala Ruler.

Who can declare it

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

Every mandala 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) (6 lines of game script)

has_tgp_dlc_trigger = yes
government_has_flag = government_is_mandala
house ?= {
	has_house_head_parameter = can_use_plunder_for_contribution_cb
}
is_house_head = yes

As written in game/common/casus_belli_types/tgp_eastasia_wars.txt.

Defender (allowed_against_character) (1 line of game script)

government_has_flag = government_is_mandala

As written in game/common/casus_belli_types/tgp_eastasia_wars.txt.

Cost to declare

Costs 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 (15 lines of game script)

piety = {
	add = {
		value = 100
		desc = CB_BASE_COST
	}
	multiply = common_cb_piety_cost_multiplier
	if = {
		limit = {
			scope:attacker = {
				is_story_godking_trigger = yes
			}
		}
		multiply = 0.25
	}
}

As written in game/common/casus_belli_types/tgp_eastasia_wars.txt.

Source: game/common/casus_belli_types/tgp_eastasia_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.