Eating out in the US today seems like more of a luxury than films and literature set before about 1980 or so tend to portray it. They seem to imply that it was much more common to eat most meals outside the home. My first question is whether or not that's an accurate portrayal, and my second is how much more affordable it would have been to do so.
The impression that people used to eat more meals in restaurants than they do today is not accurate.
In 1900, 10% of meals were eaten outside the home in the USA. By 2010, that had steadily risen to about 46%.
Eating out has gotten relatively cheaper (as has all food) over time. In 1900, the average family spent about 42% of their income on food. By 2003, that was circa 13%. 7% of the average income is spent on food in the home, and the other 6% on food outside the home (fast food and other restaurants).
So, generally, it is much cheaper to eat (as a % of income) now than it used to be, both in the home and at restaurants. Food has become more affordable, and restaurants have become more affordable. (Although, one can, of course, find restaurants at many different price points and degrees of affordability.)
This doesn't totally answer your question, but there is a section in this recent Planet Money that addresses price changes over the past 50 years. It includes some discussion of restaurants.