> How cheap does it have to be before it is successful?

A good question, but the fact is that many cannot afford sufficient food. A quick search turned up this article from 2014 at the top (and many others too):

Hunger in America: 1 in 7 rely on food banks

http://www.usatoday.com/story/news/nation/2014/08/17/hunger-...

1 in 7 Americans is around 45 million people (I know that's taking the headline at face value, but clearly there are many people who need it).