Considering the amount of research you did (this looks very impressive!), I imagine you've already come across Grain into Gold. If not, I've found it to be worth the purchase price. It offers several different prices for each item (close to the source, in a nearby city, in a far away city) which would lend themselves to a spreadsheet solution like yours (just toggle that variable for a given city/region, possibly with a percentage modifier to allow a region's products to be of a different quality than the default). I keep those tables in my DMing binder for any unexpected purchases that aren't covered by the system; their gold coin/silver coin/copper coin prices translate well enough for my needs in 5th edition. If nothing else, their presentation framework may inspire you in how you want to evolve the next step of your own model. Thanks for adding your work to the pool of DM resources, and good luck with finding your sufficiently holy grail!