You flatly don't know whether it's economical or not, you're just assuming it is, about a building that doesn't ven exist yet. The number of variables that can lead to a construction project ending up efficient or not, or economical or not, is too large for anyone to easily predict before a foundation is even laid down. Given the amount the developers presumably plan on investing in somethig this large, I'd assue they took a close look at economic viability.