Only people stuck in exploitative situations would make $350/mo for full time work in the United States, but I wouldn't be surprised if the number of people living on ~$350/mo (including rent) is in the six or seven figures.

The economic inequality between the poorer parts of the South, Appalachia, urban projects, etc and wealthy cities can be staggering. The latter subsidize national infrastructure so it's hard to find a town with extremely unreliable electricity like one might find in South Africa, but that's it - we've even got similar segregated parallel economies.

For example, Sudhir Venkatesh from UofC did a study (mentioned in the Freakanomics book) on drug dealers and their income: he found that the rank and file made less than half of minimum wage, working out to under $800 a month full time. Now spread that across the family members that the dealer has to support and $350/mo per adult is not unrealistic.