BoltBus, from what I understand, is run by Greyhound and currently operates like megabus and has service between Vegas and Los Angeles.
I just checked boltbus.com for seats for New Year's Weekend and leaving LA on the 30th I found seats for $15, and returning on New Year's Day in the late afternoon seats were $20.
Here's the piece I did for megabus that appeared on my TV show: http://alanbestbuys.com/id276.html
Boltbus is supposed to be like megabus.