Airfare is notoriously difficult to predict, but some loosely defined "rules" have emerged that make it somewhat easier to make travel plans. One is that one-way airfare is almost always more expensive than a round-trip ticket. While this seems to defy conventional wisdom -- after all, you're getting two flights for less than the price of one -- airlines have relatively solid reasons for pricing flights this way.
Business Travelers Pay Higher Prices
Business travelers' airfare is typically round-trip and almost always purchased by a corporate travel department, which is generally more concerned with finding the most convenient flying times each way rather than the lowest cost. As long as there is a large market for non-discount round-trip airfare, airlines have little incentive to offer lower round-trip prices.
One-Way Airfare is Inconvenient for Airlines
Airlines prefer that travelers fly round-trip simply because it is more convenient for their scheduling systems. Travelers who purchase one-way flights with the intention of purchasing another one-way ticket for the return journey make it difficult for airlines to make airplane staffing decisions and set flight schedules ahead of time.
High One-Way Airfare Helps Eliminate Scheduling Problems
It is common for passengers who need only a one-way ticket to take advantage of the lower round-trip fares, then simply discard the unused ticket portion. This is against airline ticketing rules, but it is almost never punished. When someone purchases a one-way ticket at a higher price than the cost of half of the same round-trip journey, the airline is able to adjust flight schedules without losing money -- whether the passenger decides to book a one-way return or not.
One-Way Airfare is More Convenient for Passengers
If you're not sure when you'll be returning, purchasing a one-way ticket seems logical. However, since the airline can't be sure you'll use them for the return trip, their one-way tickets are more expensive as a way to recoup the potential losses of you as a return passenger. You end up paying more for the convenience of deciding at your leisure exactly when you will return rather than making the decision at the time of booking.