This is all great stuff. Even if the mundane details of how much a farmer can make if he has an apiary never crop up, it's a) fun to know, and b) a potential source of inspiration and details for a game.
So okay, we've got a general idea of how much money different tiers of society make, and how much some standard goods cost. And we understand that the human population is much lower than it is today, population densities are lower, and so the ultimate potential of society is lower. In a real Medieval-esque setting, certain things may be possible without being broadly implemented.
For instance, medieval people had the technological knowledge to make crude steam powered devices (I seem to recall even way back at the Temple of Zeus, the doors were steam-powered), but they lacked the critical mass of resources and population to make it worthwhile to develop a railroad. It was simply infeasible, because they didn't have enough people to mine the necessary quantities of iron and coal, nor to construct standardized train cars.
Magic items, then, I see like technological novelties. Hell -- I admit my knowledge of electrical history is limited -- but it was potentially possible to get enough hand-made galvanic cell batteries to build an electric fence, though the cost to keep it working would have been overwhelming.
For this reason, as we develop further details, they will be heavily influenced by just how magic works in whatever magic system you prefer to use. If you can violate conservation of energy, well, all bets are off; if instead magic doesn't create energy but just converts it from sources that wouldn't work in the real world (e.g., goats' blood, diamond dust, prayer), we can at least begin to have a rational magical economy.
Now, the core D&D magic system doesn't concern itself with where the power of magic comes from, so -- like the details of peasants' and merchants' lives -- we can come up with all sorts of flavor without changing the game mechanics.
So what do you say? Should we gloss over the 'how' of a magic missile? We just assume wizards have figured it out, that they paid some metaphysical price for the physical power they wield? Maybe that's why they get fewer hit points. It's body-based fuel, or perhaps they draw mana from other planes, and doing so taxes the body. Either way, they can get their magic combat tricks. We just need to figure out a reasonable value.
Healing wounds. Creating light. Empowering soldiers to fight on. Creating barriers to keep out pests or thieves, or keep in livestock. Translation. Communicating with animals, with nature. How valuable are these things? Should it be based on how useful they are to society, or how hard they are to do? (And if it's the latter, so much depends on what magic system you're using.)
(Alternately, perhaps in another thread, I'll try a different approach. Rather than trying to figure out everything at once, we could start from the beginning. You've got a primitive stone age culture, just starting to work out barter and writing, and they have access to simple magic. What do they do with it? How do they value it?)