What does the Border Raid against the Roman Empire casus belli cost?

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

Casus belli · Invasions

Border Raid against the Roman Empire

The war is called “Border Raid against …”

Border Raid against the Roman Empire is a casus belli in Crusader Kings III (invasions). 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 prestige. The war goal is worth 100% war score. Patch 1.20.0.3, from game/common/casus_belli_types/07_ep3_wars.txt.

Quick answers (3)

What do you get for winning a Border Raid against the Roman Empire 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 against the Roman Empire casus belli cost?: 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 invasions war (the group's allowed_for_character) (7 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
NOT = {
	government_has_flag = government_is_theocracy
}

As written in game/common/casus_belli_groups/00_casus_belli_groups.txt.

Attacker (allowed_for_character) (22 lines of game script)

OR = {
	title:h_roman_empire ?= {
		is_title_created = yes
	}
	title:h_eastern_roman_empire ?= {
		is_title_created = yes
	}
	any_player = {
		any_owned_story = {
			type = ep3_story_cycle_restoring_rome
		}
	}
	is_ai = no
}
top_liege = {
	NOR = {
		any_owned_story = {
			type = ep3_story_cycle_restoring_rome
		}
		is_roman_emperor_excluding_byzantium_trigger = yes
	}
}

As written in game/common/casus_belli_types/07_ep3_wars.txt.

Defender (allowed_against_character) (9 lines of game script)

is_landed = yes
top_liege = {
	OR = {
		any_owned_story = {
			type = ep3_story_cycle_restoring_rome
		}
		is_roman_emperor_excluding_byzantium_trigger = yes
	}
}

As written in game/common/casus_belli_types/07_ep3_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/07_ep3_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 (1 line of game script)

prestige = massive_prestige_value

As written in game/common/casus_belli_types/07_ep3_wars.txt.

Source: game/common/casus_belli_types/07_ep3_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.