I suspect part of the value the bidders were considering was the PR value of having given so much to a charity *and* the PR value of having access to the book. Quite a few HP fans probably visited the Amazon site just after the purchase (last month?) to see the photos Amazon put up, deliberately blurred though they were, and some of them may have spent some money while they were there, or at least acquired a favorable impression of Amazon for sharing as much as they did. (There were a few grouches who wanted more, of course, but I think the fans were glad the winner was someone who wanted to share any visuals.)
And even though Amazon can't publish the contents at this point, they may have a foot in the door to negotiate the rights to bring out a version later. That might have factored into their bid, as well.