The good folks at Samizdata.net know economics and movies.
And lo, the Two Towers two-disk set was mine, as I'd predicted, for a mere £11-99, provided I spent fifty quid on other Tesco items. Oh please, I never get out of there for less than a full ton (£100) these days, what with nappies, slim-line tonics, and Atkins' diet steaks. So laughing all the way to the till, with a trolley load including two small steaks valued at my saving of seven pounds, I inwardly praised Adam Smith and the mysterious workings of the free market, before I bore the precious item home.