Keep in mind that there was a time where these games were made at home and sold in ziploc bags - and there's plenty of desirable stuff in that category including the aformentioned Akalebeth, the first Ultima (which I had, and - believe it or not - lost), Mystery House etc... (those are all Apple 2 games, not x86 -I lump them all together). These titles all command top dollar because they truly are insanely scarce.

On the other hand, there are games that are unbelieveably rare on PC that you couldn't sell for $5. It all comes down to historical significance and desirbility in my opionion. Games that command both categories fetch the really big $$$. I have seen many games on my want list go for hundreds of dollars. I swear you can have a giant bidding war break out on the most random titles in the PC world just because you and some other guy in Cleveland are the only two that remember Datamost and you've both been waiting 3 years for that boxed copy of Zorro.

AFAIK there's no NWC gold cart equivilant in the PC world, but I wouldn't be surprised if one of the ultra rare classics starts to get there eventually. There are only so many copies of the original Zork left... and it's a real piece of gaming history.