Now... to expand on the "value" idea...
Self-referential self-deceiving gobbldygook, I say. "What's a coin buy?" "A coin's worth." Yeah, no shit it buys a coins worth, how much is a coins worth worth? The problem is the answer given does not promise to trade a set amount. It would be a clear promise if instead it were one bar of metal. Or one crop. Relying on the coin's definition to define it is somewhat messed up. It makes room for the size of the unit of measure change, thus destroying its value as a tool for measurement. Maybe not such a good thing.
It might be messed up, but it is how things work RL, and if we are going to have coins do what they did IRL in DF, we're going to have to understand this and model it.
You raise excellent points, though. The part I bolded highlights an especially important concept. Ultimately, there has to be some way to define value in the game. There needs to be some sort of static unit of measure of value somewhere.
It's important to understand this, though: although coins are essentially a tool that tries to provide this standard measure of value, ultimately, coins or any other form of currency (fiat or commodity moneys!) are commodities like anything else, and therefore their values are subject to the same things everything else is.
So, if coins can't be used as a static unit of value measurement, what can? This question is absolutely key. I think someone's going to have to invent an arbitrary value unit somewhere down the line, but I don't think currency is the level it should be done at. I think setting coins with fixed value would make that aspect of the game all the more subject to exploit and meta-gaming. Not to mention it just wouldn't be an accurate representation of coins.
Where should this arbitrary unit of value be introduced? I think it should be introduced on the individual dwarf level. The following quote says why:
Everything is worth what its purchaser will pay for it.
Publilius Syrus
(~100 BC)
In other words, the value of something is subjective, its definition different from person to person depending on a variety of factors. But what are those factors and how can we quantify value?
I've got some ideas that we could maybe build upon and serve as an example of what I mean by "factors" and how to quantify value:
For each item, each dwarf has a numerically represented perceived value. The higher the perceived value, the higher the number for that item. This item could be anything. This number could then be used for calculating judgments made by dwarfs concerning how much to pay and with what, or how much to price something, or whether or not to agree to some agreement, or something like that. This could affect things on an individual and larger level (supply and demand!). There are several categories of value:
Sentimental
Utility/situational
Quality
Cultural/societal
(maybe others?)
Sentimental value would be affected by association with memories or other individuals (positive or negative), material preferences, personal beliefs... what others?
Utility/Situational value would be affected by necessity for daily life, immediate necessity (like life-or-death situation sort of immediate), necessity compared to what it's being traded for (if applicable)... what else?
Quality value would be affected by aesthetic appeal, workdwarfship, material... hmmm...
Cultural/societal/situational value would be affected by cultural norms, other peoples' perceived value of the item (especially important for currency!!!), social considerations (like if Urist wanted to buy the item just to spite Thikod, for example)... what else?
All of these things could somehow be represented numerically maybe. I dunno. Perhaps this is a pipe dream, but I think this is sorta the way I'd go about things.
However, I am no sociologist, economist, nor psychologist. Perhaps I should read up and do some research on the matter. There might be some helpful stuff out there...