Codeshares aren’t always at the same prices. The carrier that operates the flight, is usually the cheapest.

Not necessarily. Just today I booked my flight for Vegas later on in May. I had been checking for months on both Orbitz and UAL.com. My flights have USAirways flight numbers, operated by United, booked on UAL.com. The same exact itinerary, booked as regular United flights (not the US codeshare) also on UAL.com, was double the price. I paid $177US.