A couple of months ago we got a good deal at the Motel 6 in Mommoth, I think it was $86. It was an on-line only pre-register offer they were having. It's been a while and Mammoth is going into peak season now but but you might get lucky.
I'm on a list somewhere and get offers all the time from this site, you might look around here. Put in the timeframe you are looking for and see what comes up. It will list what they have available starting with the cheapest.
http://www.cityconcierge.com/