I doubt Commorragh would have a single currency, it is too big and the Dark Eldar society too fractured into rical factions. I could see a Kabal setting up their personal currency that they pay their warriors with and have merhcants in their subrealm accept. Say, coins that can be traded for X amount of slaves or equivalent value in goods. It would make transactions easier (since you don't have to haul arounf thousands of slaves for larger purchases or try to find a specific item the merchant wants in return for the goods), and would also allow the Kabal to pay their followers without actually spending resources as they could easily print out far more coins than they have slaves, assuming most people would buy things with the coins instead of cashing them in right away and everybody didn't decided to cash in their money at once (which is rather unlikely). Eventually you'd probably end with a Dark Eldar equivalent of a stock market crash, tho.
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Tougher than wet kittens, with armour stonger than the dampest cardboard, we are coming for you!
Kabal of Drowned Hope/Cult of the Fatal Kiss/Dark Labyrinth Coven.