With the LA based cities (LAX, BUR, ONT, SNA), I always found LAX to be the cheapest on southwest flights in the past. Depending on destination, I have seen the price gap between LAX and SNA sometimes around $150. As a matter of fact, sometimes when southwest will give you special deals, SNA is almost always excluded and always include LAX as a source/destination point (I'm not sure about the ONT and BUR since I don't have any reason to fly those airports).