Quick calculation tells me that each Astral Ward armor is ~40g a piece. The price is indeed steep for an exotic gear. But you can't tell whether the overall price for the legendary piece will be higher than the previous legendaries. For the Raid heavy armor chest piece it costs ~325g, for the PvP heavy armor chest piece it costs ~370g and for the WvW heavy armor chest piece it costs ~380g. So just 40g added to that cost for the PvE legendary armor isn't that much in my opinion, considering the fact that pve players can do their collections whenever they want without the hassle of finding players to play with. And if my guess is correct I think the relation of Astral ward armor collection to the Obsidian Legendary armor will be same as the WvW armor where you need to unlock the exotic version to unlock the Ascended precursor version for the Legendary and will not actually have to complete all of the 18 pieces to get started with the legendary.