What is and isn’t a promotion is hard to define, but a Physicists answer would be that if the cost of the ticket is less than the fuel cost of adding ~100kg load (a person plus some cabin baggage) to the flight, then the airline is making a loss on the transaction.
There again, there will be late/early “red-eye” operations which the airline is required to perform to get the plane to the right airport for the start of the next day’s operations, and I vaguely recall that a very lightly loaded plane is actually less efficient and therefore it would be worth the airline paying for a few passengers to fly as what a mariner would call ballast.
I was once invited to move up from economy to business class after the plane took off (along with the other four passengers) because the airline would save fuel that way, and we’d snooze more comfortably. (The same almost empty plane did an almost-vertical take-off after leaving the ground halfway along the runway. It went through the cloud-base before the end of the runway).
You could try flying from Warsaw to Wroclaw on Ryanair for $5 round trip (regular fare, not a promotion). Warsaw to Gdansk is priced similarly.
Internal flights within Cuba are heavily subsidised by the Cuban government.
Here is an image I took at Holguin airport of the sales desk for flights to Havana. The flight costs $187MN, which is $6.67CUC or so (the form of Cuban currency that can be converted into other currencies). This works out at £4.46 or €5.33 (about $6 USD).
Wizzair has many flights in Europe under 15€ if you’re lucky, depending on demand. Most of the cheapest flights are between a Western and an Eastern European country (for example, Germany and Romania)
One example:
https://wizzair.com/#/booking/select-flight/DTM/CLJ/2016-10-29/null/1/0/0
Depending on the destination and how crowded the day is, it can reach almost down to 10€ or can get above 100€.
Turkmenistan heavily subsidizes domestic flights on the state airline. The Turkmen Airlines site does not list domestic fares, but according to this article (undated, alas), the regular fare for Ashgabat-Balkanabad is 38 Turkmen manat (TMT). That’s around US$10.80 at official exchange rate (3.5 TMT:1 USD), or under US$9 at the black market rate (4.2 in Jan 2016, probably even weaker now).
While not giving the cheapest flight in the world it is possible to find the cheapest flight from any given country to anywhere using skyscanner, however, you could go through all countries yourself. To do this you can select select from a country to everywhere, and for the month select “cheapest month”.
Completing this for the UK gives from London Stansted to Szczecin (Poland) for £12 (~16 USD) return with Ryanair. When departing on the 22nd of August 2017 and returning on the 23rd of August, other dates where available.
Edit: mts has found flights from WMI to GDN on 15 Sep (Warsaw to Gdansk) for PLN 9 (2 EUR/1.76 GBP)
A return flight from Westray to Papa Westray is a mere £21, or £10.50 per leg. It also happens to be the shortest scheduled flight in the world. Prices haven’t changed since at least 2013 and tickets are available on-the-spot.
I’m not sure if any other route can beat that price while being just as consistent.
AirAsia regularly discounts their flights to next to nothing for a (very) few seats.
For example, KL to Alor Setar on Valentine’s Day 2017 for ~$2.45 USD (actual price is in Malaysian ringitts, of course).
They do charge a credit card processing fee for most cards, so it will run you about the cost of a big Mac (in Canada or the US).
Credit:stackoverflow.com‘