Ryanair. .
I did a number of Stansted to Prestwick moves for 50 pence all in "just for a laugh"
I got a return from Stansted to Copenhagen for 2 pence pre tax etc in 2001, but of course that was when Copenhagen was apparently relocated to a a disused air base in deepest Sweden. All good as I needed the track.
My best value for the family was in 2002 when we flew Stansted to Dusseldorf for 8 pence all in, 2 adults and 2 kids. Of course Dusseldorf was actually another air base in the middle of nowhere, actually near Weeze, again all good for new track.
We were actually staying in Belgium for a week, so the cost of an extra hotel in Aachen en route to the land of beer and waffles effectively took the cost we would have paid on Eurostar and to be honest was pretty much done for a laugh, because we could.
We came back on Eurostar as flights back to the UK from everywhere were ridiculous, but that's how Ryanair made the loss leading tickets pay. I always came back from Glasgow by train for that reason.