I don't know where you live, but I live in Mountain View, Calif.
Here's how I do it.
Expenses: I spent half a year living in my car. I now live on a couch in a garage. I pay $600/month. I have a high-deductible health plan. It's $70/month. I have a cheap car with minimal insurance. I'd estimate gas/insurance/maintenance comes out to $200/month
Living: Free food is everywhere. I go to a lot of meetups, food is often donated by corporate sponsors. Free internet is everywhere. I work out of the Mountain View public library during the day.
Income: I'm living off savings right now. When it comes down to the wire, I'll do a quick contracting job off elance.
I watch my expenses very, very closely. As long as you are young and in good health, this is sustainable for quite some time.