Using metro in Caracas (Venezuela) is for free now.
With sky-rocketing inflation, printing the tickets costed more than the ride so they made it free.
As of Jan 2018 exchange rate of USD-INR, Delhi Metro has lowest fare of just 10 rupees or 16 US cents which I think is the lowest or second lowest in the world.
You can take the London Underground for free between terminal 5 and terminals 2/3 at Heathrow. You will need an oyster card or contactless payment card though.
A ride on the Tashkent Metro in Uzbekistan costs $0.18, so it’s definitely one of the cheapest
Also you can enjoy cheap transportation in Iran, Like:
Prices in March 2015.
Some Metros aren’t on PriceOfTravel.com list. Others are out of date. The list has been updated since Mark Mayo’s answer but still outdated again quickly:
I’d like to add a second answer, because it’s one that is cheaper but not generally accessible to do individually as a traveller.
“The Pyongyang Metro, in North Korea was designed to operate every few minutes. During the rush hours, the trains can operate at a minimum interval of 2 minutes.
It is also one of the cheapest in the world to ride, at only 5 KP₩ (about $0.03 USD) per ticket.”
According to PriceOfTravel.com:
*all prices converted into US dollars in mid-November, 2010
Price ranges reflect shortest to longest rides in most cities. Tourists are most likely to pay the lowest price.
I’ll stop there, as they just get more expensive after that.
So according to this, Mexico City prices have gone up to US$0.24 now, so your best has now lost out to a few other cities, and now Caracas would probably take the prize.
Credit:stackoverflow.com‘