I just finished reading Board Enterprise's Grain into Gold: A Fantasy World Economy. It is a well thought out explanation of agricultural industry for a pseudo-realistic Middle Ages setting. The author, John Josten, uses a sound methodology; he bases his calculations on the "gold standard" of wheat. Seeing as how wheat is a fundamental food for survival and nearly all rural industry (of the time) goes towards its production, this makes for a very realistic and logical economic construction. From there, an entire rural economy is crafted with considerations for transportation to market towns and beyond.
Overall, this pdf provides insight into how each product is created and what would influence its price. The author presents a fast and simple methodology for incorporating a wealth of variables into a working economic model. For example, take growing wheat. A farmer starts off with a certain acreage of crop, then accounts for a certain % yield and then a required % that has to be set as seed for the next year. Then, the wheat has to be milled with a certain efficiency, minus the cut a miller takes. Then consider taxes and fees, and you have a logical yield and price for wheat that a farmer would see.
Yet, the wheat doesn't stop there. The miller has a portion of flour and, hopefully, the farmer has some excess. So how does that move through a pseudo-historical economy? Well, the miller prices his flour in order to secure a certain profit or daily wage. From there, the flour can be made into a wide array of food products, with each craftsman boosting the price according to the profit or wage they want to make and the complexity of their trade. So there are price points for wheat and its derivatives at nearly every point of production, whether you would want to buy wheat from a farmer, flour from a miller, bread from a baker, pastry from a chef, or a meal at a tavern. Of course, taxes and transportation sit over all of these price points, simulating common sense demands for moving goods to markets and beyond.
So iterate on that basic idea for other products and you get the cost of meat, cloth, beer, and even metals and precious gems. The finished product of this system creates an enormously helpful baseline of prices all relative to grain. With a spreadsheet, all of these prices can be instantly flexible according to how a harvest runs in a year, or based off of different products, or tweaked according to varying effects of each factor or refining step.
At the end of the book, a table of prices is presented along with standard yields of different crops and food animals. This supplement is valuable for introducing a methodology to establish a credible economy, rather than just using the fiat and arbitrary numbers that most games incorporate into their item lists. This will be a great resource when I get to the economic sections of this Early Modern Campaign.