You probably won't get to leave the day you like. I had to do a bit of digging at the site but I could find $1 trips easily. Even the most expensive trips I found still looked pretty cheap. There is also a fifty cent (.50) fee or something but who cares? This is dirt cheap! I can't get on a local bus for a buck! Amtrak is like a $100 from LA to Vegas.
Not that you have to go to Vegas. Megabus also goes to San Diego, Oakland, San Francisco, San Jose, and soon Phoenix. Vegas is more fun though :D
[Source: LA Times]