I've just been thrown for a loop when I've realized that price adjusted for inflation isn't really as comparable as I thought it was.
I've been looking at historical restaurant items at http://menus.nypl.org/ and have been trying to get a sense of how the prices compare to today's. I've originally just been plugging in prices to the rate of inflation. So while a plate of clams at $0.15 cents in 1890 is only ~$4.50 adjusted for inflation, it doesn't seem that bad. But when you factor in that an average person was making what seems at best only around $10,000 in even today's dollars, it seems to be much more a chunk of disposable income.
I'm not sure where to look for a rough equation to put all of this into perspective. How would I best say "an average plate of clams that was $0.15 in 1880 would actually be comparable to paying $__ today"?
And going further than just the 18th century—when I have apples-to-apples U.S. dollars—how could I relate the price of a menu item in, say, Pompeii or 16th century France?
The problem is not so much accuracy, but rather, what kind of comparison you are interested in making. All of the following questions will have different answers, some subtly, some radically:
"How much would an identical meal have cost then, vs now?"
"How much would the closest available meal have cost?"
"How much would an equivalent meal in terms of social status have cost?"
"How many hours would you have had to work to buy this meal?"
"How many hours would have have had to work to produce this meal?"
"How much gold/silver/whatever would I have to sell to buy this meal?"
"What other consumption would you have to have foregone to buy this meal?"
"What portion of a person's income would they have to pay to buy this meal?"
"What percentage of the total national (or even global) income would the cost of this meal represent?"
Ask yourself which of these you are interested in. None of them is the one true, correct answer, since there is no such thing as a true, correct answer. Human values do not transport across time and context unchanged. But some comparisons may serve your purposes better than others. (See my previous post here for more detail.)