Blockbuster are just starting to stock 3D blu-rays and this appears to be a good deal:
£16.99 at HMV.com/play.com
£15.85 at zavvi etc
£15.93 at amazon
Stonking review from High Def Digest too: http://bluray.highdefdigest.com/3745/monsterhouse_3d.html#Section3
Not sure how long it will stay at this price but it was only £12 today.