Cheap for most people reading HN, but unaffordable for many.
> It's doing very well indeed if you compare it to any other system tried for producing food
Yes for producing (AFAIK) but not for distributing. I believe, but have no research to back it up, that countries with better social safety nets have less malnourishment than the U.S.
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).
Making steak cheaper than it already is isn't going to make any difference.