The two best DVD purchases I have made (value vs. cost) would have to be The Good, The Bad, and the Ugly and Boondock Saints. Both great movies at only $10.
Other DVD's that I have gotten that are pretty well worth the money I paid for them:
Band of Brothers
LOTR: Fellowship of the Ring (super duper but not extra-special edition)
Blue Max (though this doesn't have any extras beyond the trailer)
The Recruit (surprisingly entertaining, if a tad predictable)
Black Hawk Down (extra-geeked-out version - 3 discs full of goodies)
Dogma (Special Edition)
Donnie Brasco (awesome movie - fuggedaboutit)
L.A. Confidential (a definite must see)
Spy Game
This is Spinal Tap (two movies in one - don't believe me? Watch it with the commentary on)
The Thing (still one of the creepiest movies I have ever seen)
Tombstone (Directors cut - Val Kilmer as Doc Holiday = cool)
The Usual Suspects (if you haven't seen it, Gabriel Byrne is Kaiser Soze - oops, nm)