I think I can fairly call all of the following films classics. They're going for a song on Amazon at the moment.
Dangerous Liaisons – $8.96, region free. Features a commentary by Christopher Hampton and Stephen Frears, the writer and director of the film.
Chinatown – $8,96, region free. Features a commentary by writer Robert Towne and fan David Fincher, amongst other bits and bobs.
Stalag 17 – $8.96, region free. Features a commentary by actors Richard Erdman and Gil Stratton and Donald Bevan who co-wrote the original play, alongside a couple of featurettes.
The Treasure of the Sierra Madre – $8.96, region free. Includes the two-hour documentary John Huston: The Man, The Movies, The Maverick amongst other things.
To Catch A Thief – $8.96, region free. Features several supplements including a commentary by Dr. Drew Casper, Hitchcock scholar.
Network – $6.96, region free. Features a commentary by director Sidney Lumet and a feature length documentary.
Sunset Boulevard – $8.96, region free. Features a commentary by Ed Sikov and plenty of other supplements.
Actually, paging through Amazon right now, looking at the reduced-price classics they have, I see that I'm barely scraping the surface. There's Casablanca, Shane, An American in Paris, Dog Day Afternoon, Grand Hotel, Amadeus, The Wild Bunch, The African Queen…
I'm not entirely sure if my wallet is lucky that I have all of these already, or unlucky that I paid quite a lot more for them at the time.