I wonder how many copies of this game ended up being sold?
A brand new copy sold on ebay 5 weeks ago for $40, so it appears that may be the going price for this game now (at least in new or very good condition)
http://www.ebay.com/itm/1983-A-Worlds-Fair-Mystery-adventure-game-PC-sealed-/111992795057?hash=item1a134a6fb1:g:F84AAOSw1KxXMf7n
Might be worth setting up an ebay search that notifies you whenever a new item appears. I doubt there is that much buying interest, so given some time and patience you can score a copy for < $10.