What's the deal with prices of old ID games?

Why are they so damn expensive? I don't understand it? I have been searching and looking at prices of vintage PC games, and most of them aren't really that expensive.. Even big box classics are fairly cheap, but I see copies of Doom, Doom II, Final Doom, Quake, Quake II, Wolf 3d and such going for between $20-$50 (sometimes for JUST the disc!) with boxed copies going over $100.. What's the deal with this? This was never the case with these games.. What am I missing?

It's probably because boxed copies are rare and so have become expensive as collectors' items.  There's not much demand for them these days with the old id games being sold cheaply on Steam and GOG.com.

5 hours ago, ENEMY!!! said:

It's probably because boxed copies are rare and so have become expensive as collectors' items.  There's not much demand for them these days with the old id games being sold cheaply on Steam and GOG.com.

It very much depends on the product but, basically this.

Very old copies which are no longer being produced or available directly from the developers in addition to only being obtainable from various sellers are typically going to be sold for very high prices since they end up as valuable and rare collectors items. There were a few threads around here somewhere by people who were interested in selling their boxed floppy disk copies of Doom which can easily be sold for around 100$, here's an example. The mail order version goes much higher than this apparently, close to 500$.

It's the same situation with old issues of music albums as well as those that are just no longer available for a reason or another. First presses along other limited releases cost a fortune, even if other versions might actually be available commercially, regardless of format.

Because their target audience is collectors. 

They're going to be expensive because they're collectors items.

On the other hand I've been buying boxed copies of Valve games this past year, and I've noticed almost all of them can be bought for less than 10 each and in good condition (I don't even try to get unopened). The only ones worth more are the big boxes of Half-Life 1 and the expansions.