GURPS Low-Tech Companion 3: Daily Life and Economics

[Lupo]

LTC3, p. 22, "MATERIAL COSTS", says:

All prices on the table assume ready access to the natural resources from which the materials are processed. In practice, scarcity and transport costs can drive up prices. For example (...) in the Classical Mediterranean (...) tin was [nearly 30 times the generic cost of "soft metal" listed in the table]

So the table is meant to be very generic... what other raw materials had a significantly higher cost in, say, Middle Age or the Roman Empire?
Or, is the cost table meant to be representative of at least one particular time and place?

If the answer to those question is "varied wildly" and "no", respectively, it is quite hard to use the cost table in actual play...
(note that I would be fully satisfied by approximate and vague answers, such as "in Europe, porcelain costed at least 10 times as much")