This is a one-way fare, of course.
Don’t expect any meal service, upgrades, lounge access or decent in-flight entertainment to be offered or included. Also, don’t expect to earn miles or points, and don’t expect to reach your destination anytime soon or arrive in style.
If you are truly budget-minded and you need to get from the San Diego area to the Los Angeles area - and you have time but do not want to raise that thumb and hitch-hike - then the San Diego - Los Angeles : $5.15 on public buses ~6 hours thread is for you…

