For EACH stage of the quest you have to buy some expensive vendor items, the total cost of which is roughly 35 mil. This is a set cost that won't change except due to racial influence.
In addition to the vendor items, one stage requires you to critical on a skill level 449 item, which is the Craft: Heart of Magic - Recipes - Aion available to all professions. Note that you need a critical, not the normal product. This is the costliest stage, because the required Balaur ingredients are needed for a lot of things. Based on the average price on 3 Korean severs people have calculated that the average cost for each attempt at critical is around 25 mil.
So if you're really luck, and critical on the first attempt, the entire Daevonian quest series will cost you 60 mil. An entire set of lv50 crafted gold armor/weapon will probably cost more than this amount.
To quell some of the more senseless comments I've posted a more detailed though still incomplete outline of the quests.
This post has been edited by tpg0007: 16 September 2009 - 01:26 AM