CDWow have both versions of the film which were released today, at the best prices.
The single disc version (£10.99) contains the film and minimal extras.
The double disc (£12.99, details below) contains an extra disc of special features.
As with everything at CDwow, if you have or can find some working vouchers/links then this can be even cheaper.
"Tom Cruise returns for a third outing as Impossible Missions operative Ethan Hunt, this time pitted against international arms dealer Owen Davian, played by Oscar winner Philip Seymour Hoffman. Directed by Alias and Lost creator J.J. Abrams, M.I.3 is a rip-roaring action adventure featuring locations as exotic as the Vatican City and Shanghai and a star-studded cast including Jonathan Rhys Meyers, Michelle Monaghan, Ving Rhames, Lawrence Fishburne, Billy Crudup and Maggie Q."