That's a simple question with a rather complex answer. Prof. Hugh Rockoff, Professor of Economics at Rutgers University, addresses this very question as an example on his web page. To go directly to the information he supplies, click on http://econweb.rutgers.edu/rockoff/HowMuch.htm and scroll down almost all the way to the end of the page. His figures sound reasonable to me based on what I've heard from other sources. Prof. Rockoff bases his figures on a cost of £1,500,000 -- or about US$7,500,000 -- the British Pound being about equal to five US Dollars in 1912.