Our goods are not only plentiful but cheap. I am a book addict. Yet even I am fighting hard to spend as great a share of my income on books as Adam Smith did in his day. Back on March 9, 1776 Adam Smith's Inquiry into the Nature and Causes of the Wealth of Nations went on sale for the price of 1.8 pounds sterling at a time when the median family made perhaps 30 pounds a year. That one book (admittedly a big book and an expensive one) cost six percent of the median family's annual income. In the United States today, median family income is $50,000 a year and Smith's Wealth of Nations costs $7.95 at Amazon (in the Bantam Classics edition). The 18th Century British family could buy 17 copies of the Wealth of Nations out of its annual income. The American family in 2009 can buy 6,000 copies: a multiplication factor of 350.
Books are not an exceptional category. Today, buttermilk-fried petrale sole with pickled vegetables and parsley mayonnaise, served at Chez Panisse Café, costs the same share of a day-laborer's earnings as the raw ingredients for two big bowls of oatmeal did in the 18th Century.
What DeLong does not mention is the very important fact that in pre-industrial Europe the cost of housing was, on average, only about 5% of people’s household income. (This is a thoroughly substantiated point: see this PDF for one example.) Even with real estate values plummeting, I don't think we’ll see that anytime soon.