The point I was trying to make (rather obliquely, I admit) was that as mechanical technology improves the difference between variations ceases to matter. If air transport goes from $500 to $5, compared to ground transport making an even bigger drop from $5 to $0.01, people aren't necessarily going to choose ground over air. An infinitely more efficient $0 walking option exists in both cases. They're going to chose air because $5 isn't enough to care; their time is worth an order of magnitude more than the cost of the transport.