The Karma vendors that sell exotic armor in Orr should sell the related runes as well.
Dungeon vendors do this already, and you could maintain the same karma/dungeon token ratio.
Each piece of Karma armor is 42k Karma, whereas the Dungeon armor ranges between 180-330. Runes fall in at 120.
This means the Runes could cost anywhere between 15272 – 28000 karma.