I've only read part of the thread, but I wanted to post a link to Game Ideas Unlimited: Cash, because it discusses the nature of monetary systems in some detail.
Looking at your statement of the problem, Nikolaj, it seems to me that you have most of the answer. You want your "tree fiver" to be worth about the same as 500 Euros. (I admit I don't know how much that is accurately, but it's probably within 20% of $500 depending on the present currency exchange rates. But then, I've had $500 bills in the past, mostly when life was such that we cashed my wife's something-over-a-thousand-dollar paychecks (they were not weekly paychecks) and paid our bills with cash.) Let me see if I can do the math:
- Tree Fiver=500 E
- 20 flowers per tree, so Flower Fiver=500/20 E=25 E
- 20 leaves per flower, so Leaves Fiver=25/20=1.25 E
- This part is unclear:
It are coins, or little plates with the symbols of it inscribed on it. The shapes change with the currency. 20 rounds into a square, 20 squares into a hexagonal.
I am not understanding what "It" means in this context. It may simply be a number error on the pronoun, that you mean "they" or "these are coins" However, it seems logical to assume that the 20/20/20 ratio matching the ratio between leaves, flowers, and trees means that hexagons are trees, squares are flowers, and rounds are leaves. I will assume that.
- Since Five Trees=500 E, 1 Hexagon (Tree)=100 E
- Since Five Flowers=25 E, 1 Square (Flower)=5 E
- Since Five Leaves=1.25 E, 1 Round (Leaf)=0.25 E.
- That gives you this arrangement of coins:
- Tree Fiver 500E
- Tree Hexagon 100E
- Flower Fiver 25E
- Flower Square 5E
- Leaf Fiver 1.25 E
- Leaf Round 0.25 E
It looks like your smallest coin is about a quarter; that's probably sufficient, because if you've got deals where pennies matter you're looking at commodities trading and competition. With this guide, you can approximate the cost of most available objects by comparing the price to your own reality.
The more complicated issue is the economic realities of the world. For example, if you don't have refrigeration, does that push up the price on milk because it has to be brought fresh from the farm, or does it push it down because it has to sell quickly and there is an oversupply? Are beef products the sort of thing that are cheap when they're available (because if you kill the cow you have to sell most of it very quickly) but rarely available (there's not much money in raising beef cattle, so it's only available when milk cows get too old)? The culture, technology, magic, and other forces are going to impact prices in significant ways--but that's an individual item decision. In some places, everyone eats turkey most of the time because the things are ubiquitous and you have to kill them to keep from being overrun by them; in other places, turkeys are almost completely unknown, and only the very wealthy ever can afford to have when brought from overseas and slaughtered for a meal.
The material would be a semi-precious stone, like Malachite or Amethist or something. A leaf and it's fiver would be made out of one material, the flower and its fiver out of another and the tree and its fiver out of yet another.
This is good color, and makes good sense pragmatically. That is, as a Multiverser world it gives the coins themselves possible value when taken to another world, while making sense within the feel of the present one.
I hope this helps. Really, you did all the work--I just crunched the numbers.
--M. J. Young