Welcome hespian.
The story you mention appears quite popular and has appeared in several other collections of Terry's short stories in a variety of languages including a German CD.
Referenced from Colin Smythe's site
Terry Pratchett Short Fiction and Verse
Terry himself answers the question thus...
Quote:
Tell us about "The Hades Business"—your first published story. What was that story about? Was that your very first acceptance, or were there any others before it?
Pratchett: Wrote it as a class project*, the kids liked it, sold it to John Carnell at New Worlds Science Fiction, one of the three fully-pro mags we had in those days (early '60s). First story, first sale. I've always been a bit embarrassed about that. I'm sitting here on a stack of money in a big house so I can't really complain that I did things wrong, but I sometimes wish I'd done more between then and 1982 than write a novel every five years. On the other hand, I suppose I was absorbing stuff! Oh, and getting a life and running a career ....
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* It looks like Business Rivals may have been an earlier submission to the school magazine. This would account for the differences you've already spotted.
Taken from Science Fiction Weekly interviews at
Science Fiction Weekly Interview
As for the value, it's difficult to tell how things could go. Maybe your enquiry thread on eBay will provide a better answer.