Airfare pricing is based on what the market will bear and is not directly connected to what it costs an airline to fly a person.

When making one-way fares high, airlines are considering that most of the purchasers needing a one-way fare need to travel - no matter what the price is. Leisure travelers deciding if they will go or not based on price usually don't need one-ways. So if most people who are looking for one-ways will pay however much they have to pay, why should an airline sell it for cheap?