It I'm to believe what I read on-line (which is a dubious proposition), it depends on the place. I hear that in the US, McDonald's is actually cheaper than healthy food, especially if you count calories and nutritional elements per dollar spent. In most other places around the world, this definitely isn't the case.
That said, fast food solves an important need, IMO. It's usually a very tasty hot meal you can get quickly. Which is exactly what I want every time except when I'm taking my SO for a romantic evening.
It is situationally true in the US. Inner city people generally have less access to cheap produce. People who have cars and access to the capability to buy in bulk at places like BJs or Sams also have access to cheaper food, so perversely the poorest end up buying more of their food from small shops that charge more.
That said, fast food solves an important need, IMO. It's usually a very tasty hot meal you can get quickly. Which is exactly what I want every time except when I'm taking my SO for a romantic evening.