Let's say you're writing a story set in in 1934. Your central character is Joey, a hobo down on his luck. He counts out the grimy coins in his pocket and discovers that he's got $1.16.
Not much. Poor Joey. But it was 1934. What buying power did $1.16 have in 1934, compared to today? You could do all kinds of research and figuring and come up with a guess.
But I can tell you exactly. Joey would have the buying power of $17.79.
And here's how I know. There's a web site service by the Minneapolis Federal Reserve bank. It's called "What Is a Dollar Worth?" and it can make the comparison, from way back then to right now, or the other way around, instantly. Here it is: www.minneapolisfed.org/Research/data/us/calc/
So, if you're in to time travel, and you're walking the streets back in 1934 and Joey says, "Buddy, can you spare a dime?" he's really asking for $1.53 in 2007 money.