Fixed costs are always hard to do, especially in a barter system. Sure, rules of thumb are nice, but no system other than an order entry/board of trade would come close to actually realizing prices at any given time.
Things certainly have a time value to them, but that won't account the simple fact that I
WANT something alot more than something else and will pay a premium or am flooded with other goods that I want to dump & therefore devalue. Steel might be considered the most expensive, time-consuming item in the game, but I currently have absolutely no need for it (other than to trade it away), so its value to me is next to naught (remember it has to be stored, also, and my inventory space is precious). In the end, it truly is up to the individual trade.
Plus, an industry-spec'd miner can get ore more easily than a nature-spec'd farmer, but then the farmer could make tea more efficiently, which should mitigate some of the pricing variances.
I mean, I gave Vattic such a great deal, I had to indenture all nine of my children.

But, he also made the trip to me and hinted at making future trades - all things I would take into consideration when pricing a load of goods.