xliioper

There are limited cases where this should be true. Domestically, a simple A-B-A roundtrip should never cost more than two one-way's and I challenge someone to show an example. Multi-city is a different situation as the cheapest one-way fares may often only allow A-B-A routing fare combinations.

The most common case where this is true is for trips to Canada. This is because the airlines generally price one-way fares to/from Canada for the same "dollar" amount. However, if you buy as a roundtrip, both flights are priced in US dollars. While if you purchase as two one-way's, the outbound fare is priced in US dollars while the return is priced in Canadian dollars and you get a cheaper total fare due to the exchange rate. However, there are still Canadian markets where DL has roundtrip fares published and you can get a cheaper fare by purchasing the roundtrip fare if you meet the advance purchase and minimum stay requirements. For many longer haul international destinations, roundtrip fares are generally much cheaper than two one-way's as long as you meet advance purchase and minimum stay requirements. The main cases where it would not be true is if you are book short notice trips and don't meet the advance purchase requirements of the cheaper roundtrip fares. Then you can potentially get cheaper pricing as two one-way's due to differences in point-of-sale pricing from international markets.