There are so many variables on this question. Having low income often means you can't take the cheapest route because of a higher required initial outlay.
Also, at this point, a lot of people in this thread have already put forth a lot of valid responses, like whatever you can lease for $79 a month, though insurance on a lease can be high - the one surprise with my lease was that I failed to account for the much higher insurance limit requirements when planning it out. Or, whatever cheap Sentrollavlier you find for $2000.
Honda Civic Hybrid was at the top of some list a several of years ago for lowest cost of ownership. They're pretty cheap now, but someone elsewhere in the thread mentioned battery failures. I chose my CR-Z in part because of cost of ownership. I get pretty good mileage and I don't spend much on maintenance and renewables.
Here's the oddball.
I fear I bring this car up too often in what car threads. I do this because it has a lot of unique qualities, and it's forgotten.
Mitsubishi i-MiEV.
Wait, hear me out.
See that car up there? That's an actual car for sale on Autotrader right now. That car is a 2012 model with about 12,000 miles on the odometer. It's almost new. You should be able to get reasonable used car financing, since it's only a 2012. It uses tiny, skinny, cheap tires. It gets something like 119MPGe. YMMV, but I can refill it completely for something like a buck twenty. The first scheduled maintenance is at 240 months, when you change the transmission fluid. 240 months is twenty years. There are no oil changes. You can't really fail to maintain it, because it doesn't really require maintenance.
How much is the asking price for that car right there?
$6,500.
Now, hold up. You might be tempted to tell me that you drive a hundred miles a day. You tow a trailer. Yeah, this car isn't for you. It's not for everyone. If you're renting in the city, you might not be able to plug in at home. If you're renting, you might have to charge at 110v, and your daily mileage needs might not be met by the overnight charging time available. You might live in an extremely cold climate, where your range is too significantly curtailed. If you have to drive really far, haul big things, or do any of the things this car can't do, it's a terrible choice.
But if you don't? If you're living in a trailer two miles outside of town? If you have two incomes and two cars, and one of them doesn't have to do long trips? If you're a retiree on a fixed income who runs errands around town?
I recognize the i-MiEV is only acceptable in a narrow set of circumstances, but in those circumstances, it should represent a solid choice.