Two economics professors, one from Maine and one from Illinois, have provided an invaluable service to those wanting to measure the relative worth of things over time, from the eighteenth century to today.
Their website, http://www.measuringworth.org/, features several calculators based on a variety of official UK and US government statistics and economic indicators, including the retail price index (the cost of goods and services purchased by a typical household in one period relative to a base period), average earnings, and three measures based on gross domestic product.
One calculator allows you to learn the present worth of a past amount (for example, the cost of Big Ben, the salary of the Archbishop of Canterbury, or the price of tea); another tells you what a historic price in British pounds is worth in US dollars today (and vice versa).
A quick crunch of numbers related to my own specialty, theatre history, reveals that Lillie Langtry's £250-per-week salary at the Haymarket Theatre in 1882 translates into a whopping £126,478 in purchasing power per week today. Of course, exorbitantly paid performers like Langtry were by far the exception and not the rule.
This site should come with a Surgeon General's warning about how addictive it is.
Shown here: The Royal Exchange and the Bank of England in an undated photo, c. 1890.