@ nefadol : I love your Avatar! *-*
Thanks! I'm on a coloring manga kick lately. ^_^
I doubt Fei Wong is a vampire, he has a very long lifespan like Clow Reed, but not a vampire. Maybe he drinks wine because he's an alcoholic like Yuuko? Although Yuuko's smoking may be for the same reason that Subaru and Seishirou smoke in X, it increases their powers. Perhaps alcohol works in the same manner?Â
On what Kamui and Subaru paid, all we know is that it's probably expensive. It also depends on how often they can travel, Seishirou had a limited amount of times he could hop. I'd say their blood is most valuable, given that it can save a life and Yuuko would have charged more than Kurogane could have given to do the same.
Either Yuuko meant the same price to Subaru, or that she'd need payment equal to the wish as when she granted their wish to travel to other worlds. Basically she's saying "I don't need to explain myself, right?".