How to answer this more logically...
They are sold for that much because that is a merchants job, to buy low and to sell high. Anyone can make grapejuice, but not everyone does. Anyone can cook, but not everyone does. Repeat this logic as necesarry, and infer that not everyone wants to get on everyday on a merchant and stock up on items they are out of, equally not everyone wants to buy 100 days worth of supplies at a discounted price and store it. We play for the moment, and when one is on a knight and want dicounted meats fast, they go get some from the folks at payon dungeon (used to be more merchants in the orc dungeon too).