It definitely varies from route to route. This summer I booked two one-ways LAX-JFK and JFK-LAX (first AA second DL), and the pricing was identical to booking a round trip.
For a trip next month a one-way DFW-LAX on DL was only $70 cheaper than LAX-SEA-DFW-LAX (connecting in MSP for the legs to/from DFW)...