I just bought a brand new sealed original big box release of Quake 1 for AU $12 with free postage. That's slightly cheaper than a sealed one I saw that cost well over $100 (someone actually bought it for that much, for some reason).
i'd probably buy a cheaper, used copy to play and make use of, then find a sealed copy and keep it around for display. may be a bit pricey to buy sealed copies if you just wanna open them and use their contents in the long run... well, not in this case, at least. :P