For the most part, airlines price oneway fares at significantly more than 50% of the return airfare. In some cases it can even be more than 100% of the return airfare. The reason airlines do this is because by definition 2 oneway tickets are more flexible than a single return ticket. With 2 oneway tickets you can avoid minimum stay requirements, book circle trips (A to B to C to A), and other useful stuff.
If travelling oneway, or on an itinerary where a series of oneway tickets may be sought, it is a good idea to check return airfares even if you think you won't need the return flights.
- The return fare might be cheaper due to airline pricing policy, or alternatively you may be able to buy a higher fare category for similar amount as a oneway. A higher fare category means easier and cheaper (or free) to make changes, and lower or no cancellation fees.
- The extra for the return flight may be relatively low if taxes are higher in one direction than the other (eg taxes departing Australia are much higher than arriving, similarly for UK).
- Some countries require a return or onward ticket in order for visitors to enter.
- Even if the return fare is slightly more expensive than one-way, it may be a cheap insurance policy against the need to buy an expensive last minute one-way flight back (eg if your circumstances change or a family or other emergency).