In my class yesterday, we discussed whether eating a healthy, vegetarian diet was financially possible for lower-income people in the U.S. I mentioned that there are a variety of legumes which are very high in protein (soy, lentils, etc.). These tend to be inexpensive relative to the cost of meat. A quick look online reveals that a pound of chicken, for example, costs about $1.50 and a pound of black beans costs about $1.12. It's worth noting that beans and legumes are not as high in calories, saturated fat, and other things which can have a negative impact to one's health.
But this doesn't answer the question at hand. Can a poor person in the U.S. afford to eat a healthy vegetarian diet? I think it's certainly difficult if not impossible to eat well on a very low income. To put things in context, the poverty line in the U.S. in 2014 was set at $23,850 for a family of four and the average family spends about 10% of their income on food. Though lower-income families likely spend a higher percentage than this, it seems that such families simply don't have sufficient resources to eat well--regardless of whether their diet includes meat. But the point I tried to make in class was that it's more expensive to eat meat than it is to eat no meat and eat vegetables, legumes, etc. instead. In addition, one can get a great deal of protein and other nutrients one needs to be healthy without eating meat.
Readers of this blog might find this website and this one of interest.