What does the Bloc Expansion casus belli cost?

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

Casus belli · Subjugation

Bloc Expansion

All Under Heaven

Bloc Expansion is a casus belli in Crusader Kings III (subjugation). If the attacker wins: The attacker will seize the contested Titles for the …, and decide how to distribute them. If the defender wins: The attacker … humiliated. Declaring it costs prestige. The war goal is worth 1% war score. It needs All Under Heaven. 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 Bloc Expansion war in CK3?: The attacker will seize the contested Titles for the …, and decide how to distribute them (patch 1.20.0.3).
What happens on a white peace?: The defender will defend their lands.
What does the Bloc Expansion 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 will seize the contested Titles for the …, and decide how to distribute them.

White peace

The defender will defend their lands.

Defender wins

The attacker … humiliated.

If the war becomes invalid

Bloc Expansion 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) (5 lines of game script)

has_tgp_dlc_trigger = yes
is_landed = yes
highest_held_title_tier >= tier_county
tgp_soryo_or_bushido_trigger = yes
exists = house.house_confederation

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

Defender (allowed_against_character) (13 lines of game script)

is_landed = yes
highest_held_title_tier >= tier_county
top_liege = scope:attacker.top_liege
NOT = {
	this = scope:attacker.top_liege
}
government_is_japanese_trigger = yes
trigger_if = {
	limit = {
		exists = house.house_confederation
	}
	house.house_confederation != scope:attacker.house.house_confederation
}

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

When it can start (valid_to_start) (3 lines of game script)

scope:target ?= {
	is_landless_type_title = no
}

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.