Due to UA's routing rules, and the fact that they DO have the lower fare buckets on p.s. flights, one can fly p.s. via connections for next to nothing at times (and certainly far less than O&D pax).

Earlier this year, I flew IAD-LGA/JFK-SFO-SMF-LAX-JFK-IAD as a mileage run for under $180, and upgraded both p.s. segments. You can't beat that!