It is a matter of "willingness to pay" and
market price segmentation, which ultimately comes down to supply and demand.
By the way, the pricing of a product has nothing to do with the cost of manufacture; this only applies for commodities. For all other products, a good pricing strategy will involve estimating the demand/price graph for the consumer group in question and optimising the price-volume point to yield maximum profitability, either through very high prices (but few sales), a very high sales volume (at dirt cheap prices), or more likely, some combination of the two that comes out on top of either end. Of course there is a question of supply costs underneath that, but it is not the primary concern of the pricing analyst.
Okay so that's all very simple. But hang on: obviously different people will place a different valuation on the same products. How can you charge one person more than the next person for the same product? (Even though the two people have different valuations of it?) That is called market segmentation.
Business travellers [I am not talking business class, I am talking about those travelling for work] can, in general, afford to pay a lot more for their tickets but there aren't enough of them to fill up the plane and run a profitable operation. On the other hand you have very price sensitive leisure travellers who will spend hours hunting down a fare that saves them ten Euro cents. You could lower the price for everyone, but then you are leaving money on the table from those who are willing to pay more for your product.
So how do you sell the same product to two different groups of people at two different prices?
The current way to apply price segmentation to the market is a mixture of flexibility on the ticket; advance purchase requirements; and minimum stay.
Business travellers often (not always) require a good deal of flexibility on the ticket. If my meeting in Paris finishes early, I want to go home early and see my family. If my meeting in San Francisco next week is delayed by a few days, I don't want to get there early and waste time, when I have work to do in my office in England. Obviously, if I am a holiday traveller, I don't care, I have no meetings, I will just find the cheapest ticket that lets me sit in the sun. Therefore, flexible tickets are considerably more expensive.
Advance purchase follows the same rules. Perhaps tomorrow a client will call me and ask me to come to Osaka on Monday morning. Most people do not go on holiday at the drop of a hat.
Finally, the important one: minimum stay requirements. I do not want to stay away from home at the weekend. No one pays me to work on the weekend? However, if I am on holiday, I probably will be willing to stay for a Saturday night if the price is a lot different.
Therefore, tickets that enforce a minimum stay (of say three days), or more likely a Saturday night, are much much cheaper than tickets that do not.
However, if you price one way tickets at the same price as a discount return, you enable all the restrictions I have mentioned to be defeated by the purchase of two independent, one way tickets. Therefore, one way tickets are inevitably priced at the far top end of the pricing spectrum, to prevent their use in this way.
One way tickets should almost never be purchased by anyone, except those with substantial corporate discounts. There is almost always a cheaper way of doing what you want.
Now the LCC market has shaken this up particularly within Europe and domestic flights within the United States. BA, for instance, now sells European fares at the price of a half-round-trip. However in the business class cabin this logic is not applied. I am aware that AF is sticking to its guns and refuses to sell discounted oneway European fares.