While on holiday, I wondered how much it would cost me for a one-way economy ticket for the little 1hr hop from AMS back to NCL. About £100 for the return so I guessed about £50.
They wanted over £400
Most (or certainly some) carriers heavily discount economy fares, so the majority of pax don't pay the full economy fare unless they want lots of flexibility.
These discounted fares are almost never offered on one-way flights, even if it appears that they are on the booking engine.
As
virginblue states, people buy a round trip and don't use the return sector. But don't do this in reverse: if the outward leg isn't used, the carrier will usually cancel the return sector.
No frills carriers may have different arrangements.