What he said. Plus some were only available in Japan. Making them rare, further increasing the prices. They may not always be the best sounding ones but that goes for recent remasters too.
Personally, I'm only after the music. Although the ones I have the value have increased considerably, which is nice.
Some sound fantastic, some sound flat and dull. You need to really research the titles before buying. The promo boxes are also collectible and can go for big $$$ as well.