Obviously the Golden Outfit is the most expensive since you need to pay 500kk for the outfit and 250kk for each addon, totalizing 1kkk.
Mage Outfit Full (Male) or Summoner Outfit Full (Female) is really expensive since you need a Ferumbras' hat and it ranges from 600~680kk
Battle Mage Outfits are really expensive since you need 5 Sturdy Books for addon 1 and it is 15kk each and 20 epaulettes for addon 2 and each one costs around 20~30kk.
Elementalist Outfits Full requires a elemental spike and price ranges from 85-100kk
I think the Makeshift Warrior outfit is kinda expensive aswell since you need a Plan for a Makeshift Armor and it ranges from 15-20kk.
Perhaps there is a more expensive outfit than Makeshift but I can't remember right now.