As I said before, if you want to look in detail at one mans take on economics in a medieval fantasy rpg, check out the Merchantyle article here
http://home.comcast.net/~harnmil1/downloads.htm
As to the why’s of this discussion, you need to broadly consider society as well as economics. I don’t profess to understand economics, but lets start with the basics. We’ll use Sid Serf and Baron Corleone as our examples.
Everyone needs to eat. Without food, even Baron Corleone will die. Sid and the Baron can hunt for it, but game gets more sparse, and they need to travel further to find it, so Sid and the Baron fight over hunting rights. The Baron wins. Sid needs food, so he begins to manufacture it. He collects eggs from his hens, lets his pigs forage in the forest, ploughs fields and plants grain. The barons says, ‘hey, that’s my forest.’ By now he has built up a gang, he needs a lot of food. Sid and his friends come to an agreement with the Baron. The Baron will stop ANYONE ELSE stealing Sid’s food, provided Sid gives the Baron a cut. The Baron takes as much as he can get away with. He does not take everything, because if he did Sid and his friends would starve to death. Then the next year the Baron either starves, or works the fields himself. Somewhere here the words get changed, and Sid finds that some of his pigs, sheep and fields now ‘belong’ to the Barons. Practicaly, there is no change, Sid still looks after the same amount of stock & crop, but now, not all of it is his own. That, basically, is the feudal system.
Then, of course, you get to add the specialists, Stan the Smith and Mick the Miller are both useful to Sid. Stan makes a new iron plough and hoe, which lets Sid plough more land and grow more grain. Mick’s mechanical expertise grinds more grain more quickly, so Sid (and the Baron) get a little more food. They both need to pay these guys, Mick simply takes a cut of the grain he grinds, Stan too will take payment in kind. The Baron also likes Stan because he can make swords and armour. This lets the Baron put the squeeze on his neighbour, Baron Hardup. Faced with a powerful force, Baron Hardup negotiates. Rather than lose everything, he agrees to let Baron Corleone (who has now decided that he wants to be known as Earl Corleone) take a cut of his revenue. Earl Corleone must protect the specialists, because without them, the whole complex edifice collapses.
The above misses a lot of other specialists, the clothiers & dyers, who take the fleece from Stan’s (and the Earls) sheep and make clothes. The tanners and leatherworkers who, among many other things make vellum from the sheepskin so that the Earls clerks can make an account of his lands.
Note that there is no reason why any of these people should ever handle cash. Some will, but Sid probably never sees any, he has food on his table and if someone causes trouble for him, he expects the Earl to ‘send the boys round’ to sort the trouble out. The Earl will, probably do this, because if he doesn’t, the rest of his serfs will start saying, ‘why are we giving him our grain, if he won’t help Sid?’
The ‘merchants’ are the next specialists. These are the people who deal in cash and relative values of commodities. They move wool from the Earls hills full of sheep and in return supply the Earl with iron, because there are no mines in his fief. If wool is in short supply because of an infestation of sheep ticks, prices rise. Good for the Earl? Not if, as a consequence, the price of that new suit of clothes he wants doubles. He doesn’t pay? The merchant stops dealing with him, and tells his friends ‘watch out for that guy.’ Those merchants who do deal with the Earl offer him less, and often ask for payment in advance. Bottom line, the whole system only works so long as the majority of people have full bellies.
Oh, and Gizmo, you’re right, there has to be an accepted base price. Selling something for less than it cost for materials and manufacture is only an option if you’re starving, mad, or want to go out of business. What we call consumer protection legislation dates back, at least, to the Assize of Bread and Ale, which set the price of ale and the weight for a farthing loaf of bread.