The cheapest is amazon.co.uk. I just purchased the single blu ray and it is region free as plays perfectly on my Sony s570. It is $11.64 plus shipping to the USA. The BD has ALL of the features of the US release but without the extra junk like the board game, magazine, etc. It a Warner Bros release to the UK. I am assuming this is the same as the US release. Why pay 30 bucks or more when you get the movie plus all of the video extras for around 20 bucks or less. Makes no sense to me.
Wait - they did the same thing with Wizard Of Oz & Gone With The Wind... those were priced around 60-70 bucks each and the single discs are now around 15-16 bucks each. Wait - there will be a disc-only release, guaranteed.