Are UK ebook bestseller prices trending down?

I've started to do some historical analysis of the ebook price data I've been collecting.

In honour of the fact that I'm now going to be blogging for the Bookseller magazine, I decided to look at the UK market first.

(I'm not sure whether I'm going to be able to add the images to the Futurebook blog; check the original at if necessary)

So I took the most recent Futurebook Top 10 from the Bookseller, and looked back to see what the price you'd have paid for this set of 10 over the last 3 months. Here's the result, a definite trending downwards. 

ebook Price Watch: the Futurebook Top 10 from June 10th 2011

What this doesn't show, however, is the cause and effect - did these books get in the Top 10 because their prices were cheaper than the competition? or did the bookstores/publishers drop the prices because the books were selling so well?

I also looked at one other set of books, the 8 books which made up the Richard&Judy selection for Spring 2011.

ebook Price Watch: Richard & Judy Spring 2011

This doesn't show such a definite trend. but the average cost of the books starts above £5 and ends below it. I'll do the same for the Summer selection; it will be interesting to compare the two sets of books.