Usually RTW fares are cheaper than just a number of one-way tickets. However, if you are willing

- to combine different (non-allied) carriers, and perhaps
- even different travel classes

a number of one-way tickets can become more economical. A friend of mine, for instance, booked the cheapest airline one-way for each route and used business class only for the really long and overnight segments. Like this he saved a couple of thousand dollars compared to a business class RTW ticket.