This is a very easy process. For example, we have already determined that the cost for a pound of manufactured pottery is 18.685 c.p., or approximately 19 c.p. per lb. We need only determine how heavy a given object made of pottery weighs, multiply that against our ratio and then judge for ourselves how carefully worked the object is.
Let's take something very simple: an earthenware pot, the sort of object that might be used every day and be churned out in large numbers, for ordinary use around the house - such as would be used for flour, cookies, buttons and so on. We already know that the pot is fired, as this was part of the process that increased the cost from mere clay to being pottery. We can add that the capacity of this pot is about a pint.
Let's establish the weight of the pot at ½ a pound. Let's also establish that because the pot is very ordinary, the workmanship is 1.0. This makes the cost of our pot equal to 18.685/2*1 (in excel calculation), or 9.343 copper pieces. For the players, we will round that out to 9 c.p. That's a reasonable price; an ordinary household may have half a dozen of such pots. An orc lair might have a hundred or more.
By why limit ourselves to ordinary workmanship? There is such a thing as 'art pottery,' some of which is spectacular in design and color. We can well imagine establishing a set of perameters for pottery of greater workmanship, based on quality of clay (the best material is reserved for the best pots), additions such as handles and lids, color, sculpting, quality of sculpting and so on, so that workmanship for a given piece can be rated as x 2 (fair), x 4 (ornamental), x 8 (fancy), x 16 (artistic), x 32 (quality), x 64 (excellent) and x 128 (exquisite). Thus, a truly exquisite pot (still the same weight) can now be rated at 1,152 c.p. (or about 6 g.p.).
That's nice - but the party isn't likely to get excited about 6 g.p. (though a household of such objects might make someone take notice). Still, we can always enlarge the pot. Our one-pint pot is about 7 inches tall (23 cm); how much would it be worth if it were, say, 14 inches tall?
In such a case, we need to multiply all the dimensions by 2: height, width and thickness of the material (though the latter may not be necessarily true - it is up to us). This would mean our 14-inch tall pottery vessel would be multiplied by 8 times and be worth 48 g.p.
Now, that's something. But let's not stop there. Let's suppose the party stumbles into a big lair and finds that the chieftain sits between two massive pottery urns, each 49 inches tall (about a meter and a half, for all us moderns who aren't familiar with imperial measurements). And let's say that the workmanship is magnificent (x 256). How much would they be worth? From a usual game standard, we're just guessing - but now we don't have to guess. 49 inches in height is 7 times our original pot's size; we're increasing the size in 3 dimensions, so the urns are 343 times as large, or about 172 lbs. each in weight. The base cost, size multiplied by the value of ordinary pottery, is 3,204 c.p. The workmanship, however, increases that value to 820,346 c.p. In my game, that's 4,273 g.p. Those are pots that are very definitely treasure!
By this method, we can logically determine the precise value of any pottery object in a way that is consistent with all other pottery objects. We can do the same with metal goods, horses, leather work, ale or anything we like. We need only establish the difference in "workmanship" between an ordinary ploughhorse and a heavy warhorse to give us a range of possible prices all based on the same original price we generated for horses.
If this isn't precise enough, we only need to create another set of references for a particular kind of metal, horse, leather or beverage, running through the same system we've already built, to produce another set of numbers we can use to price things.
And always remember, the price for one market in our world always has the potential for different prices, both higher and lower. Where is the best place to sell those big pottery urns?
How heavy is something? We have the whole internet to use as a judge. In fact, we can find specific objects on the internet and say, "such and such a pottery jug, in this picture here, weighs this amount and I'm calling the workmanship this." In reverse, the player can present an object, describe its dimension and ask what the price would be and we can work it out in a few seconds, without worrying about whether or not it would 'break the system.'
For those who would be interested in going on the 'net to find the weights for objects, I have a hint: search for 'shipping weight.' Most sites will be very unclear about how much a depicted object weighs, even on sites that are selling things - but those that ship objects are always interested in specifying weight; often distinguishing the difference between shipping weight and 'actual weight.' This can save a lot of hours wandering about trying to determine how much a telescope, a ship anchor or a pair of boots weighs. For historical objects, such as swords, museum sites are often very helpful.
I have priced more than 1,300 objects in this manner; my blog has many examples of equipment tables, some of which do give the exact weights of objects. As I write this, I am upgrading my prices table, reorganizing it to make it easier to expand and to more deeply adjust for the price of an object. I do plan to make this table - in excel - available for users, but only those prepared to invest $10 per month in my patreon account (see link for details). Sorry about that.
We're not done, however. This more or less describes the skeleton of the whole system. Having described it thus far, I can now begin detailing specific elements of the system, such as how to build structures that will calculate trade distances between multiple cities, organize hundreds of different references at the same time, splitting and handling more specific references, calculating out things like different products, adding wages and performed services (from getting a shave to hiring a berth aboard a ship) and so on.
We have a lot of things yet to expand.
See Trade System
A note about Patreon
After some reading on the site, I've learned about a disappointing practice that only seems human. Fans pledge an amount for a given creation, only to withdraw that amount when the artist sends the creation before waiting for the pledge to materialize. Not at all unexpected.
Therefore, I'm willing to give a 'taste' of the content for my pricing table for anyone who donates, particularly since it is in a state of rebuilding (and therefore is lacking a huge amount of information); rest assured, the long-term gains from pledging will far outweigh what can be gotten immediately.