The situation is the same for DVD's, CD,s and Books; any kind of media format that is made for a while then discontinued, i.e. goes in and out of print. Just start looking things up on Amazon sometime and you'll run into the phenomenon. A CD is 14.99 when in print and as soon as it goes out of print it's 40.00-100.00 or more. Amazon lists sellers by price and when collectors buy the "lower" priced items you may find a CD or DVD priced over a thousand dollars as I have a few times. These are just scalpers, like ticket scalpers. When Proctor & Gamble stopped making Crest Vanilla Mint toothpaste they were scalping it on Amazon for 25.00 a tube. They'll scalp anything.
Lately studios do a lot of "on demand" DVD's so you may try a studio site. Amazon has begun doing some CD's by demand, usually those at high prices, though I don't know if they are doing DVD's. I streamed Scaramouche tonight as a rental, but could have bought a cloud copy for 9.99. You can also just be patient. Over the last few years I've gotten most of the overpriced CD's on my list because eventually someone will offer it at a normal price (not low but not scalper-priced). You just have to check periodically.
A week later: I finally checked Amazon and everybody seems to be selling the DVD for around $15.00 so it must be back in print again. Need I say get it now or forever hold your peace.