When several hundred of the most powerful people alive gathered in Idaho this month for the Allen & Company conference—the invitation-only retreat the press calls summer camp for billionaires, held July 7 through 11—the tech founders who now outnumber the media moguls arrived dressed to a man in the same costume, with nary a suit in sight. The uniform is a plain gray T-shirt that looks like it costs $12, but in fact costs $300, and that gap—betw…