I found this out 6 years ago when my wife and her sister were doing a European tour. They extended their stay and wanted me to book them from Paris to Madrid. I tried all the train deals online and was shocked at the pricing compared to earlier trips we had made. Using a European travel site, I booked an Argentine airline for the flight, about half the cost of the train. Using the European online travel services also saved money on the hotel. Some of the flights and hotels were not available on English language services, but a Euro is the same in English or Spanish.
The Brits will tell you to take the train anyway, but... "no pressure."