Beethoven CD price variation

I have just bought Beethoven's Late String Quartets, played by the Takacs Quartet - a 3CD set on Decca. I bought it from the only independent shop in Chichester - Bastows Classics, for £15.99, which is the list price including the usual dealer markup. While browsing in HMV a little later, I found the same CD for £31.99. How can there be such variation? And isn't it good that the specialist shop, complete with people who know what they are talking about, beats the giant hands down?

Nigel