GoEuro has released a transportation price index for train, bus and air travel for number of countries.
It is not really a surprise that the cheapest flights per kilometer flown are either in developing or large countries where the average sector length high.
You can access the GoEuro’s web page for this study here.
You always need to read the fine print:
Numbers in the index have not been weighted against GDP per capita or PPP of the countries involved, therefore these results do not reflect wages, infrastructure or quality in the transport industry.
Prices can vary greatly depending on mode of transport and location. We took an in depth look into the price of various transport options around the world to compile an ultimate price index. We found the cheapest regular flights between the most populous cities in each country and then calculated the average price per 100km.
This is some nice food for thought, but you have to take it with a grain of salt as well. Obviously, the flight prices in small sized countries such as Switzerland are high per kilometer flown due to the sector length while in the US they are far more reasonable (LAX-JFK is 2500 miles and you can often get it for few hundred bucks or less).
The flight prices are low in India, Thailand and Malaysia, but people there earn a lot less as well compared to more developed nations.