I mean, if you're going to measure the worth of something by its rarity/the difficulty of procuring it, it should be more expensive than platinum. 2 Soul Coins in the whole of EA? Currently worth 10 gold each.
But as soon as there's greater transactional demand ...
There's actually three. aside from the one the Teifling can give you, there's two in the crypt, although one of them is missing a text description and won't stack with the other two.
As of patch 5, there is now only one in the crypt (the trapped room), the one that used to be in the room with the book of dead gods is now missing.
As to value, I remember seeing once (don't remember which patch) a value of 100 gold for the coins. I have not checked in patch 5 though.
