The value of money, then and now

I was following links on Wikipedia and ended up reading about the Alaska Purchase. This led me to the Louisiana Purchase, and a claim by the author (or editor(s)) of the article that the $15 million purchase price paid by the United States to France was the equivalent of $278 million in 2007. That struck me as rather low, to whit: in 1863 a private soldier in the Union Army was paid $13 a month (room, board and clothing were provided). In 1985 a U.S. private soldier (yours truly) was paid $750 a month (room, board and clothing still provided). That is an increase of 57 times in number of dollars paid, versus just 18.5 times for the supposed value of the Louisiana Purchase.

As with most Wikipedia articles, there were links to the references used. In this case, http://www.measuringworth.com/uscompare/ which is a facinating site on its own. It is certain the author of the Louisiana article used the Consumer Price Index comparison. But if you plug in my Civil War to 1985 numbers, the unskilled wage comparison is a better fit. In fact, if the numbers are accurate, Union soldiers were better paid than their 1985 counterparts! The comparison we need to look at in my mind, though, is the the 'relative share of GDP' one. If we plug the Louisiana Purchase numbers in and look at that, we get a modern value of $430 billion!

I guess the whole point of this is, when someone compares historic prices to current prices, you need to ask, "What is your frame of reference?" Because depending on how you look at the problem, you may get a misleading answer.