I have watched fares up and down the West Coast for over 10 years and while all airlines have to be competitive with each other I have found on average Alaska charges higher fares when they are the only one's who offer non-stop service. Average cheap tickets between LAX/SEA have always been in the $198-$200 range. The fares that have gone up are the SEA to SFO/OAK/SJC. The average fares were right around $100-$130. You are lucky to get a fare now for $150!!!

Currently Alaska does offer the lowest fares to Southern California from SEA but they have longer stay requirements and are only valid for certain days of the week.

If you want to fly from SEA to LAX for one day, or travel on Friday or Sunday, Southwest is the cheaper way to fly.