What does the Border Raid casus belli cost?

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

Casus belli · Struggles

Border Raid

Border Raid is a casus belli in Crusader Kings III (struggles). If the attacker wins: You gain Gold in proportion to the Development of the target County, which will decrease. A Building may also be destroyed. No Titles will change hands. If the defender wins: You must pay Gold reparations for attempting the raid. Declaring it costs piety and prestige. The war goal is worth 100% war score. Patch 1.20.0.3, from game/common/casus_belli_types/03_fp2_wars.txt.

Quick answers (3)

What do you get for winning a Border Raid war in CK3?: You gain Gold in proportion to the Development of the target County, which will decrease. A Building may also be destroyed. No Titles will change hands (patch 1.20.0.3).
What happens on a white peace?: You fail to complete your raid.
What does the Border Raid 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

You gain Gold in proportion to the Development of the target County, which will decrease. A Building may also be destroyed. No Titles will change hands.

White peace

You fail to complete your raid.

Defender wins

You must pay Gold reparations for attempting the raid.

If the war becomes invalid

The … has ended: the defender no longer … the County.

Who can declare it

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

Every struggles war (the group's allowed_for_character) (8 lines of game script)

can_use_struggle_cbs_trigger = yes
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) (7 lines of game script)

any_character_struggle = {
	involvement = involved
	has_struggle_phase_parameter = unlocks_border_raid_casus_belli
}
NOT = {
	government_has_flag = government_is_landless_adventurer
}

As written in game/common/casus_belli_types/03_fp2_wars.txt.

Defender (allowed_against_character) (11 lines of game script)

any_character_struggle = {
	involvement = involved
	has_struggle_phase_parameter = unlocks_border_raid_casus_belli
}
OR = {
	character_is_land_realm_neighbor = scope:attacker
	AND = {
		exists = scope:attacker.liege
		character_is_land_realm_neighbor = scope:attacker.liege
	}
}

As written in game/common/casus_belli_types/03_fp2_wars.txt.

When it can start (valid_to_start) (6 lines of game script)

exists = scope:target
NOT = {
	scope:target.title_province = {
		has_province_modifier = recently_looted_modifier
	}
}

As written in game/common/casus_belli_types/03_fp2_wars.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 (26 lines of game script)

piety = {
	value = 0
	if = {
		limit = {
			scope:defender.faith = scope:attacker.faith
		}
		add = {
			value = minor_piety_value
			desc = CB_SAME_FAITH_COST
		}
	}
	multiply = common_cb_piety_cost_multiplier
}
prestige = {
	value = minor_prestige_value
	if = {
		limit = {
			scope:defender.faith != scope:attacker.faith
		}
		add = {
			value = minor_prestige_value
			desc = CB_COST_DIFFERENT_FAITH
		}
	}
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/03_fp2_wars.txt.

Source: game/common/casus_belli_types/03_fp2_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.