Join Date: Jun 2013
Well, It's fine for me to allow to field an Iyanden themed army and to play it officially (thinking in tournaments). Background, illustrations, miniature photos, rules, special characters... everything it's right too and don't matter the prize (GW, take my money!)
It's true that It could be cheaper than its base codex (Eldar).
If I'm right, looking the eBook description on iTunes it has the equivalent of more than 100 printed pages. Then think that having the same amount of printed pages, it's "logical" that GW put the same prize that a codex.
Surely GW will give us supplement codex for factions that varies from the usual army. I can imagine some examples like: Black Templars as supplement codex from Space Marines codex; Maybe Death Korps of Krieg as supplement codex from Imperial Guard codex; etc.
You don't need to buy all supplements, you only will buy it if you really want to play this specific themed army.
Then, It's very expensive the supplement? Yes, it is, like codex.