This is an interesting question. I am primarily interested in liquor establishments like saloons but my research has stemmed into the broader patterns of American consumerism during the early 20th century so I can provide some perspective here. From what I have found, the American diet changed dramatically following World War I for a multitude of reasons.

My go-to "starter" text for this sort of information is a book called Daily Life in the United States, 1920-1940: How Americans Lived Through the Roaring Twenties' and the Great Depression by David E Kyvig (2002). However, it is important to note that Kyvig's text is a more generalized history of this period and does not go into great detail nor does he make good use of citations (he does a blanket citation here and there). For this question, Kyvig primarily draws from two texts: Middletown: A Study of Modern American Culture by Robert S. Lynd and Helen Merrill Lynd (1929) and more importantly, Revolution at the Table: The Transformation of the American Diet by Harvey Levenstein (1988). Both are excellent sources for the study of the 20th century and I would recommend them.

I will provide a brief list as to the reasons why the American diet changed during this period according to the summary provided by Kyvig (Pages 106-121- in no order of importance):

• During the food shortages of WW1, the US government instructed the public that they could maintain a healthy diet by eating the right kinds of foods (proteins, carbohydrates, etc.) rather than eating more food overall.

• The consolidation of major food production companies (Examples- General Foods and General Mills).

• The death of the saloon and the usual "free lunch" following National Prohibition in 1920.

• The virtual disappearance of the middle-class servant.

• The expansion of the entertainment industry (broadly construed here) as seen in places like movie theaters, community clubs, and the fascination with the auto-mobile.

There are likely other reasons why this change occurred that I am missing but in the interest in conciseness I will say that the 1920s diet can be summed up as an "Eat and run" mentality in which the traditional family dinner becomes increasingly challenged by more "simple meals." However, it is important to note here that at home cooking is never entirely replaced by alternative eating establishments like your book suggests.

Now lets look at some numbers. Kyvig states that during the 1920s, the number of restaurants tripled. Broken down this included 48% as lunchrooms, 26% coffee and sandwich shops, 11% full-service restaurants, and 8% cafeterias. However, there are several points here which need to be discussed. First, there is the interchangeable use of "lunchroom" and "cafeteria" that I see both in the historical records and from historians. For example, a lunchroom could quite literally mean a place to eat lunch (ex. a child bringing a lunch from home- common during this period) or they could be more "cafeteria-like" in their operation. Second, returning to my earlier critique of Kyvig, he does not cite a particular source or page rather he did a "blanket citation" of Levenstein and the Lynd's so we will have to assume that this source is located in either of these texts which I unfortunately cannot find in my collection at the moment. The rest of which Kyvig speaks to I have seen elsewhere in my studies so I have no reason to distrust him as a historian but nevertheless it is something I have to point out.

I do, however, have an excellent book on this subject called How the Other Half Ate: A History of Working-Class Meals at the Turn of the Century by Katherine Leonard Turner (2014:81-85) which touches on these matters more clearly. Turner starts with the premise that with the death of the saloon came to the death of the "free lunch." Interestingly, many saloons simply transformed into restaurants or other food proprietors like soda and ice cream shops. You can see this change quite literally on Sanborn Fire Insurance maps. Turner notes that there were numerous ideas for how the "lunchroom" should operate. She notes that one book in 1916 expressed that the food should be setup as a "grab joint" in which people would select their food from a buffet counter, pay for each item, return to their table and eat, and then bus their own dishes when finished (funny idea given the state of cafeterias today).

Returning to the numbers again, Turner states that the number of lunchrooms and restaurants increased by 88% during the 1920s and unlike the saloon offered universal access to women and men in a liquor free establishment. Unlike, Kyvig, Turner's source's here are quite clearly stated and accessible (data from the National Restaurant Association, US Economic data, and newspapers). Turner also notes that it was during this time that lunchrooms and cafeterias in major cities later developed into chains (like Child's Restaurant's in New York). As with before, the contents of these lunchrooms varied as some had light items like oatmeal, cereals, and pastries whereas others served a whole meal.

Answer:

To answer your question then, as you pointed out in your post to consume every meal from a fast food joint or restaurant would be incredibly expensive. The same would have been true back then to some extent as the growth of "lunchrooms," according to Turner, catered primarily to white-collar and "lower middle-class" city workers although working-class folks did have some options. There was definitely a class element here as the vast majority of these proprieties were located in business districts and heavily catered towards the young urban professionals (more than likely white collar). The location of these jobs in the inner-city and the demand required for them spurred a great deal of people to seek more accessible food options as found in restaurants, lunchrooms, and cafeterias. Of course, similar establishments opened in working-class settings like mills and factories but these were likely different than those absent of direct industrial influence. There was a unique instance of a working-class orientated lunchroom- Turner cites the lunchroom opened by Emma Goldman in Worcester, New York). Overall, while the growth in alternative eateries was significant during this time period it was never to the extent that the vast majority of people ate exclusively at these establishments as your book may suggest. Regardless of class, food cooked at home was the dominant form of consumption during the 1920s with the biggest factor being the overall cost of food.

If you are interested, I looked up what the trend is today and I found a 2018 report for US Department of Agriculture. Here is the abstract of the report:

Food away from home (FAFH) has become increasingly integral to the American diet. In 2010, the share of Americans’ food budget for FAFH—reaching 50 percent (up from 41 percent in 1984)—surpassed the share for food at home (FAH) for the first time. Likewise, Americans’ share of energy intake from FAFH rose from 17 percent in 1977-78 to 34 percent in 2011-12, with differences in growth across types of FAFH (e.g., full- and quick-service restaurant foods, school meals, etc.). Along with the demand for FAFH, availability of FAFH has also increased, with much of the growth in recent years attributable to quick-service restaurants. The growing presence of FAFH in Americans’ diets reflects changes in consumer demand and producer behavior and affects the health and nutrition of individuals over time. This report takes a comprehensive look at the role of FAFH in American diets, exploring nutritional composition of FAFH and key Federal programs that may influence FAFH. The report also discusses how FAFH choices and availability relate to diet quality, income, age, and other socioeconomic factors.

It will be interesting to see how the COVID-19 Pandemic has changed this. Let me know if you have any other questions and I will do my best to answer them!