This mission, simply put, was his "wish"; the Dragon Queen paid the bill. At least he didn't have to worry about the extravagant expenses this time.
If he truly had to build a Sky City from scratch—not even considering the lost technology that might be irreproducible—the development costs for a single spell like "Proktev's Mountain Moving," which the Devil Archduchess had used earlier to "move mountains," amounted to a staggering 2,230,000 Gold Coins. And that was just one arcane spell.
What thrilled him even more was that the "return clause" in this contract only included the thirty thousand or so Neisser souls. Was it because the wealthy Glaciya didn't care about this "Baator copper shell," something she could conjure effortlessly with her Devil Archduchess's Authority? Or had Tiamat directly paid for the materials? Amazingly, he didn't even have to return the blasted thing!
According to his plan, it was impossible to use the energy-guzzling Hell Engine for the entire journey.
