What does Cooper mean when she says"...non-nutrient foods are really, really
cheap which is why I say it's a social justice issue" (7:50 in video)

This video.

https://www.ted.com/talks/ann_cooper_what_s_wrong_with_school_lunches/transcript#t-262479

It's a TED talk.

Explanation:

Cooper means that many people are only able to buy very cheap foods, which are not nutritious. Americans that make less money are likely to have very poor diets and, therefore, more health problems than wealthier Americans because of their inability to obtain nutritious food.

Something like that.

Hope that helps!