It's doable but, as you would expect, it requires an extremely spartan lifestyle. I'm running about $500/month in SF and am no stranger couch surfing, lots of roommates, sleeping in my car, and (intelligently) fasting, etc. I could theoretically go even lower by purchasing lower quality food but I'm not willing to compromise my health.

Once you've seen the worst-scenario first-hand you realize it's not that bad, and you can shoot for the moon without any fear.