One tricky question is inflation. What’s the value of a gift made in 1985 or a fortune inherited in 1972? A number that no longer meets your institution’s standard for a “major gift” may have been much more significant at the time. There are several inflation calculators that help put historically appropriate frame around a numerical snapshot from the past:
The Inflation Calculator provides an adjustment based on the Consumer Price index from 1800 to 2003, based on Historical Statistics of the United States (pre-1975), and the annual Statistical Abstracts of the United States. It also links to other CPI-related sites, as well as inflation calculators for Canada and Italy.
NASA offers the Cost Estimating Web Site . There are a few different calculators here, using various indexes which may help you compare apples to apples and oranges to oranges: the Consumer Price Index; the Employment Cost Index, which measures changes in wages, salaries and benefits; and the Import Price Index, which measure the transaction prices of goods and services exported from or imported into the United States.
If your data gap is geographical rather than temporal, a good currency conversion site can be found at Oanda.com. From dollars to drachma, the site allows you to translate between 164 different currencies, listed by country, and to enter a date for historical conversions. For example, 100 Finnish Markka was eqivalent to $22.76 back in 1995. It can also print handy “cheat sheets” for those overseas trips. I'm still waiting for a client to send me, all expenses paid, to the Land of Euros, but hey, a researcher can dream, can't she?
Coming soon, my take on another easy-to-find, hard-to-interpret number: house valuations.