Cheap Hostels in San Francisco?

Me and friend are taking part in a camp america program this summer and are then travelling afterwards so we are VERY budgeted, esp as we are sudents. We have found a hostel for $20, which I know is very cheap, but were wondering if any1 knows of any which are a little less? We have found 1 in Vegas and a few other places but can't seem 2 find 1 for less in San Fran. Thanks alot