A while back, UA use to provide a RT break over 2 OWs if there was a Saturday night stay. That has been gone at least 10 years and most domestic RTs are the sum the associated OWs. Still occasionally RTs cheaper then the OWs and OWs cheaper than RTs do show up but likely just glitches in file fare construction.
Internationally many times there is a OW penalty (no discount OWs offered) but in some competitive OW markets you will find reasonable OW pricing.
Competitive pressure, or lack of, many times is an underlying factor.