One of my favorite Internet resources is How Much Is That in Real Money? The site provides links to a variety of calculators for determining purchasing power over time.
My old Signet Classics paperback Moby-Dick, for example, cost $1.25 back when I bought it in, I think, 1977. In current dollars, that would be about $4.42 (which as it happens is nearly the current price of the Signet Classics Moby-Dick).
Technical aside: The HMITIRM calculators are now actually maintained at a site called Measuring Worth, but I find it much easier to remember "How much is that in real money?" when I need to go there.