It's the regional publisher spitting that we got from pbooks. Usually in pbook each region has their own publisher and they do their own thing.
From amazon.com The Alexander Cipher was publish on March 18, 2009 in hardcover by Grand Central Publishing which is a subsidary of hachette
from amazon.co.uk The Alexander Cipher UK version was publish 5 Nov 2007 by harperscollins UK in paperback
So both publisher make ebook.
The US publisher decide that it would be a great idea put ebook at hardcover price.
UK one base their price on their paperback version.