So I’m not really sure if this counts as a bug or not, but if it isn’t a bug its a huge oversight. The obsidian shards in any of the Heart of Thorns maps cost 12,250 Karma plus 125 of the specific map currency, such as Airship Parts, to get 5 of them. That means it takes 24,500 Karma to get 10 Obsidian Shards, ignoring the extra currency.
This IS A BAD DEAL since you have buy 10 obsidian shards for 21,000 Karma from Tactician Deathstrider at the Cathedral of Glorious Victory. So again Do Not Buy Obsidian Shards from anywhere in Magus Falls that use Karma! (Guild Hall purchases are different entirely and so are fine, just good luck getting it)
You have been warned, at least until ArenaNet sees the mistake and fixes it.