Crygil has announced that the cost of the Argent Hippogryph flying mount will be reduced from 250 Champion’s Seals to 150 Champion’s Seals in the next minor content patch. Crygil said the change is being implemented "in order to better balance the cost of new rewards that we would like to add to the Argent Tournament."

If the price still says 250 Champion's Seals once the change has gone live, it's just a glitch and will be corrected in a future patch.