People spend what they have. There are many people who are living on $700 or less a month. I know there's a video on YouTube by cheaprvliving where Bob is interviewing a woman living in her car, I believe it's a car, on $850 a month. If someone doesn't drive a lot, parks in free places, cooks their own meals, and doesn't have a lot of bills like health insurance, credit cards, medications, or a car note, then their living expenses will be pretty low. At that point their main expenses are food, propane, car insurance, cell phone plan, laundry, and miscellaneous stuff. Plus if they get food assistance, which someone living on $850 a month should qualify to receive, their food costs might be completely eliminated.
My personal budget is more than $1,000 so I can't help with budgeting but I know a few people living on less than $1,000 a month in income-restricted apartments and they're making it just fine, even with paying a few hundred dollars in rent. If they can, someone in a vehicle can.