When traveling I am trying to avoid using the underground/metro/subway because I want to see the city I am visiting, but sometimes for long distances it is much quicker than any other form of transport.

So which city has the cheapest underground system? I'm looking for the cheapest available single fare price. It is okay if I have to use a pre-paid card as long as I can buy it right in the station.

On contender is Mexico City:

Photo credit: Peter Hahndorf

In 2008 the price for a single ride was 2 pesos or € 0.10 (US$ 0.15), it is now up to 3 pesos, but that is still very cheap.