I agree that the relative cost of travel will rise, but I am also fearful that interest in travel will fall as the next generations are more content with a "virtual" experience and less willing to commit time, money and energy to travel.
However...
Huge leaps in the ability to travel could also drastically change the game as we know it. Think supersonic planes that travel on sea water or alga found in ocean water that could fly New York to London in 2 hours for even less cost than compared to now...
Think "Star Trek" style transporter that could transport people from one side of the earth to the other in minutes...
Think underground high speed rail tunnels that cut strait across the earth to travel from North America to Asia using only the force of earth's gravity to speed up as they get close to the earth's core and then slow the train pods down as they move away from the center of the earth, in as short as a few hours...
Think of "space elevators" that could take travelers to the edge of outer space in a matter of minutes to awaiting operating space platforms or ships that could travel to the far reaches of the solar system...
All this idea seem far fetched now, but I'm sure that the idea of flying over the Pacific ocean in a plane seemed far fetched just 2 or 3 generations ago