Implicit in that statement (taken together with the comment you're responding to) is the assumption that the kinds of plants cheaper than meats and dairy are healthier than a lacto-carnivorous diet. Locally, chicken and live anchovies are cheaper than lettuce, tomatoes, broccoli, and even beans and rice when denominated in pounds, sulfur, protein, calcium, B vitamins, .... I can understand the argument for having a high level of plants in your diet, but I'm skeptical that it's cheaper to live on just plants unless you only count calories and no other nutrients.
Plus I think they were probably talking about how all of those options are vastly more expensive than living on Raman and peanut butter sandwiches, especially once you count for prep time and power costs.
