Maybe this will do it for you: https://www.bloomberg.com/news/articles/2014-03-03/advertisings-century-of-flat-line-growth https://www.bloomberg.com/news/articl

Maybe this will do it for you: https://www.bloomberg.com/news/articles/2014-03-03/advertisings-century-of-flat-line-growth https://www.bloomberg.com/news/articles/2014-03-03/advertisi.... Ad spend as a % of U.S. GDP remains relatively constant since the 1920s.