There are numerous copies of movies distributed in varying incarnations...When I bought my copies of the Jurassic Park movies, I intentionally looked for the versions which had DTS as there were instances of both...some with and some without DTS, but the prices were the same...unless you are buying a "Special Edition" or somesuch which comes with an extra disk with all the featurettes on how it was made and a lot of sometimes worthless addtional information...If you do some additional research, you should be able to find most any movie (if it was offered in DTS) at a price comparable to most others...Double in price sounds like a rip-off...
Definitely don't settle for the prices at Worst Buy or Short Circuit City or other Mass Marketer...you can do better.
My .02,
WhatFurrer
