I dug through some of my old files. Here's what I have found so far:
* City 2.26 (c1998, Hartley Patterson) - Outputs very basic city information given certain inputs. Not really what I'm looking for, but available here: http://www.daisy.freeserve.co.uk/city.htm
* D&D Economic System Simulator (c2001, Matt Warren) - This is the spreadsheet I remembered. It takes into account a simplified supply/demand factor for commodities (including certain intangibles), and a difficulty factor in bringing specific goods/services to market, to produce an adjusted price for each item. Even better, because it's in Excel it is fully customizable (e.g. which commodities play into the price of each item; changing what commodities are tracked; adding new items) so can be customized to any cash-and-carry system. It doesn't look like this file is still available (though I do have a copy).
* A Magical Medieval Society: Western Europe (c2003?, Expeditious Retreat Press) - Has about 9 pages whose simulation appears to focus on availability. Modifies DC of availability check by base (i.e. book) price of item and on the size of the community in which the purchase is attempted. Modifies die roll based on modification of offered price (e.g. a character who offers more than book has a better chance of finding the item). No discussion is made of using a skill check for this roll, but that is an obvious modification (Streetwise/Haggle, anyone?). This would be an easy mod to the Economic System Simulator, above.
I quite by accident ran across another Expeditious Retreat product "A Magical Society: Silk Road." To quote their sales pitch, this product "details great overland trade routes, how they work, and how to create them in your game." Anyone have this product and want to discuss it a bit?
I'm always interested in more information, so if you've got something new (or at least new to me) let me know.