What do you get for winning a Iberian Reclamation war in CK3?

You vassalize the defender (patch 1.20.0.3).

What does the Iberian Reclamation casus belli cost?

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

Casus belli · Conquest

Iberian Reclamation

Iberian Reclamation is a casus belli in Crusader Kings III (conquest). If the attacker wins: You vassalize the defender. If the defender wins: … will remain Independent. Declaring it costs piety and prestige. The war goal is worth 80% 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 Iberian Reclamation war in CK3?: You vassalize the defender (patch 1.20.0.3).
What happens on a white peace?: The defender will remain Independent.
What does the Iberian Reclamation 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 vassalize the defender

White peace

The defender will remain Independent

Defender wins

… will remain Independent

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 conquest 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) (8 lines of game script)

any_character_struggle = {
	involvement = involved
	is_struggle_type = iberian_struggle
	has_struggle_phase_parameter = unlocks_iberian_reclamation_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) (7 lines of game script)

NOT = {
	any_character_struggle = {
		involvement = involved
		is_struggle_type = iberian_struggle
	}
}
fp2_iberian_reclamation_involved_county_trigger = yes

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

When it can start (valid_to_start) (7 lines of game script)

scope:target = {
	any_de_jure_county = {
		any_county_struggle = {
			this = struggle:iberian_struggle
		}
	}
}

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 (40 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
		}
	}
	add = {
		value = 0
		scope:defender = {
			every_realm_county = {
				limit = {
					any_county_struggle = {
						this = struggle:iberian_struggle
					}
				}
				add = fp2_expel_interloper_county_cost_value
			}
		}
		desc = CB_COST_IBERIAN_RECLAMATION
	}
	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.