Add to that; solar powered planes. The 'Solar Impulse 2' has just completed a 56 hour trip from Hawaii to San Francisco. It is the first of its kind to be able to fly day and night without the need for any fuel.
At current, speeds are rather limited; 28 mph during the night and up to 56 mph during the day when the sun is beaming down on the solar plane. But that doesn't detract from the amazing feat that this is. With the prospect of air travel not requiring fuel, we can only assume that air travel may not always be expensive as it is today.