I'm of the opinion that you shouldn't trust black market prices to be "base" prices because a) much rarer for the frags you want to show up, and b) limited number of frags to buy at one go (compared to treasure of the sage). As such it makes sense that this would be compensated somewhat by "discounted prices".

Obviously this is speculation and unconfirmed, but my point is still don't rely on black market to be a good comparison point.