Here in San Francisco, we have a plethora of Farmers Markets. The thing about produce is: once harvested, the clock starts on it going bad. So the farmers have a ticking timebomb on their hands: if they can't sell the produce at the end of the day, (in most cases) it will go to waste. Some of the enterprising ones take a trip to Chinatown and offload their leftover produce for pennies.
So when I was in a situation of hardship where I was living on a few dollars per day, I would go to the farmers market near closing time, and over time developed friendships with them, so they would sell me their remaining produce for dirt cheap. I would take it home, turn some of it into stock and freeze that.
Over several months, I survived on about $100/month in groceries.