This is a calculator which tracks how amounts of money in different time periods correlate in terms of the number of stamps they could buy.
It has been claimed (via mph) that since the post office cannot legally make a profit, its stamp prices will accurately reflect its operating costs, and therefore stamp prices are an easy way to track real, effective inflation, which is much higher than the Gummint admits. I don’t know whether that’s true, but if you want to assume it is, here are the consequences.
I wrote this late last night rather than going to bed on time for some reason.