Megabus:
http://www.megabus.comLimited departure times and stations means that you'll be at the whim of their schedules, but for ~$10 each way, it's pretty hard to argue with the price. Services California, Nevada and Arizona only for West Coast.
Current Times: (Multiple schedules available, please check on megabus.com)
Saturday: Arrives at Los Angeles at ~3:30 PM
Sunday: Departs from Los Angeles at ~7:00 AM
Should be used in conjunction with...
Van Nuys Flyaway:
http://lawa.org/vny/flyAwayInfo.cfmLeaves every half hour, except for early morning, which is hourly. The Van Nuys flyaway is only 20 minutes away from Interface and 5 minutes from FFA, and it only costs $8 each way to Union Station (or $4 to LAX).
To use with the MegaBus, take Union Station to LAX (which is 20-40 minutes depending on traffic), then transfer from LAX to Van Nuys. FlyAway buses stop under every terminal at the green "
FlyAway, Buses and Long Distance Vans" signs and are marked with their route.
Picture of a FlyAway bus