A cursory search of the waterstones ebook prices shows that prices vary between a few pence cheaper than the equivalent paper version up to a couple of pounds more expensive. Which is outrageous. Whilst there are a number of costs that are shared by paper and electronic books (cover art, editing, proofreading, royalties, etc), and ebooks do require investment in server storage, online transaction processing and small amounts of additional artwork (reducing cover art to thumbnails whilst keeping them recognisable is not easy), the savings from avoiding the printing and distribution costs should still result in significantly (very, very roughly in the 20%ish ballpark) cheaper ebooks, given equal profits to publisher and author.
Waterstones may be blaming the publishers for this, but I'm unconvinced. Personally, I blame lack of competition in the UK ebook market - you've basically got Waterstones and WHSmiths (and now Amazon), and that's about it. In the states, there are dozens of online bookstores, and given identical content they can only really compete on price. Hence, lower prices.
The authors and publishers inability to grasp the concept of global ebook rights, because they are so used to selling the physical rights on a country by country basis, are a large part of the problem here (selling the physical books on a country basis makes a lot of sense buts makes little or none for electronic - there are interesting VAT issues to overcome there, but these are hardly insurmountable). Without those artificial boundaries to sale, there would be much more opportunity for competition.
But the largest problem is still that there just aren't enough ebooks being sold for the publishers and authors to take the time and make the effort to get things right. And of course, this is a chicken and egg problem - ebook sales are being massively restricted by DRM, poor interop, lack of standards, artificial barriers to purchase and high prices in many countries. And without those sales, there's no obvious incentive (to idiot suits, at least) to fix the problems. Good old Catch 22.
I should mention the one exception to the dreadful state that ebooks are in these days (in most ways worse than 5 or even 3 years ago), which is Baen Books, an american sci-fi/fantasy publishing company that publishes all of their books as non-drm'd ebooks, and you can buy them no matter where you live in the world. And they make the first quarter of each book available for free on the web (a practice akin to the crack dealer telling you the first hit is free...). And they have the Baen free library, which is basically the same concept applied to either series or authors in general, where they first few books in a series or several of the authors individual works are made available for free to get you hooked on that series or author. Frankly, I've given up buying ebooks from anywhere else (and no, I don't work for them)