What do you get for winning a Individual De Jure Duchy war in CK3?

The attacker gains the contested titles (patch 1.20.0.3).

What does the Individual De Jure Duchy casus belli cost?

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

Casus belli · De jure

Individual De Jure Duchy

Individual De Jure Duchy is a casus belli in Crusader Kings III (de jure). If the attacker wins: The attacker gains the contested titles. If the defender wins: The defender keeps the contested titles, and the attacker must pay them war reparations. 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/00_dejure_war.txt.

Quick answers (3)

What do you get for winning a Individual De Jure Duchy war in CK3?: The attacker gains the contested titles (patch 1.20.0.3).
What happens on a white peace?: The defender keeps the contested titles.
What does the Individual De Jure Duchy 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

The attacker gains the contested titles.

White peace

The defender keeps the contested titles.

Defender wins

The defender keeps the contested titles, and the attacker must pay them war reparations.

If the war becomes invalid

The … has ended: the defender no longer … any land in the targeted duchy.

Who can declare it

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

Every de jure 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)

scope:attacker = {
	culture = {
		has_innovation = innovation_casus_belli
	}
	NOT = {
		government_has_flag = government_is_nomadic
	}
}

As written in game/common/casus_belli_types/00_dejure_war.txt.

Defender (allowed_against_character) (11 lines of game script)

scope:attacker = {
	has_de_jure_claim_on = scope:defender
}
NOR = {
	scope:defender = {
		target_is_liege_or_above = scope:attacker
	}
	scope:attacker = {
		target_is_liege_or_above = scope:defender
	}
}

As written in game/common/casus_belli_types/00_dejure_war.txt.

When it can start (valid_to_start) (33 lines of game script)

scope:target = {
	tier = tier_duchy
	any_this_title_or_de_jure_above = {
		holder ?= {
			OR = {
				this = scope:attacker
				target_is_liege_or_above = scope:attacker
			}
		}
	}
}
scope:target = {
	trigger_if = {
		limit = {
			scope:attacker = {
				is_ai = yes
			}
		}
		any_in_de_jure_hierarchy = {
			continue = {
				tier > tier_county
			}
			count >= 2
			tier = tier_county
			holder ?= {
				OR = {
					this = scope:defender
					is_vassal_or_below_of = scope:defender
				}
			}
		}
	}
}

As written in game/common/casus_belli_types/00_dejure_war.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 (25 lines of game script)

piety = {
	value = 0
	add = common_cb_impious_piety_cost
}
prestige = {
	value = 0
	add = {
		every_in_list = {
			list = target_titles
			every_in_de_jure_hierarchy = {
				limit = {
					tier = tier_county
					OR = {
						holder.top_liege = scope:defender
						holder.liege = scope:defender
						holder.liege = scope:defender.liege
					}
				}
				add = 100
			}
		}
		desc = CB_CLAIM_COST_TARGET_TITLES
	}
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/00_dejure_war.txt.

Source: game/common/casus_belli_types/00_dejure_war.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.