Generally speaking, for domestic flights, the RT vs OW costs are the same or a negligible difference. The disadvantage, as mentioned above, is if you have to cancel, you pay two change fees. There have only been a handful flights that I have seen recently that have a significant price advantage booking a roundtrip - one I remember is PHL-JFK. I find I mostly book one-ways because I don't book until I know for sure I am going, my travel can be a bit complex, so its easier to piece together one-ways, and sometimes I know the date I might want to leave but not return, so it gives me some flexibility to save some money on the flight I know for sure I am taking. Generally speaking for international travel, its almost always going to be significantly cheaper to book round trip. Google flights and matrix are good resources to play with.