I also thought that the approach he took on that one didn't make much sense. I thought the answer would have to do with how much people would pay to live in equivalent conditions. When the cambist found that the king's days were worth less, I assumed at first that this was a result of his responsibilities, not his vices.
Available in PDF here, the short story in question may appeal to LW readers for its approach of viewing more things than are customary in handy economic terms, and is a fine piece of fiction to boot. The moneychanger protagonist gets out of several sticky situations by making desperate efforts, deploying the concepts of markets, revealed preferences, and wealth generation as he goes.