I'm a bit concerned that Amazon are offering the book at £3 off for pre-orders when the net proceeds are supposed to be going to a children's charity, it seems mean to me. I'd happily pay the full price since it's only £6.99!
Just because the net proceeds are going to charity doesn't mean that all the "extra" money from a more expensive purchase will go to charity. If it's £6.99 in a shop, the shop and distributors will still get their usual share of the price. Amazon sell a lot of books at practically half the RRP, and this is no different. Indeed, the real villains are sellers that *don't* discount it as much as they would a normal book because they want you to think that all the money goes to charity.
If I buy from a bookshop it's because I want to support that bookshop; if I want to support a charity I'll save as much money as I can on the purchase and pay the balance directly - with GiftAid etc this would definitely mean the charity would get more!
I think like most things a fixed amount per book is donated whatever the actual price paid by the customer. Amazon say £20 will be donated from the £50 of the collectors edition so I would guess around £0.70-£1 gets donated from the standard edition.
Amazon will be paying no more than £2.25 for the book if its retail price is £6.99 I can already get 65% discount off the RRP of new books, and I am a small seller. As long as I order a RRP value of £1430, then I get it. How much do you think Amazon get? And they were in the news recently for attempting to get further discount, with the threat to blacklist the publishers that refused.