On a Thursday afternoon this spring, a few dozen well-heeled travelers descended via helicopter to the Rosewood San Miguel de Allende, a five-star hotel in central Mexico. Rooms at the hotel typically cost four figures a night. Over the course of the weekend, the group went horseback riding and tequila tasting; on the final night, they enjoyed a private fireworks display. All of it was complimentary — except the flights, which cost anywhere from $7,000 to $23,000 an hour for clients of Flexjet,