[Glice]

Think of it like Warhol. He produced many copies of each of his paintings. Each wasn't a 'unique' artefact, but it was an artefact of Warhol's art.

Simple economies of scale: a single Warhol is worth more than one for which there were 50 produced. If there was, say, a single copy of the first Beatles demo, that would be worth much, much more than my copy of the White Album.

There's the popularity of the band to factor in, but the fact that Black Flag (or whoever) are now incredibly influential means that their original, limited artefacts are worth a lot of money. If their first single was a limited edition of 1, it'd be worth a lot more than a limited edition of 500.

None of this comments on the quality or otherwise of the music - it's not really about the music itself but everything that surrounds it. If you just want the songs, download them (or buy the CD re-issue), but if you like the artefact (which is still a huge part of listening to music for a lot of us), the rare 7" is much more appealing. Personally, I have a lot of sentimental value attached to certain parts of my record collection, but that doesn't quite extend to spending my arse on rarities.