[ravenfish]

Aye. Multiply weight in pounds [1] by (price of a pound of bronze[2] - price of a pound of iron[2]) to get the additional price for using bronze rather than iron. It's not perfect (for example, some of the weight of any armor is straps and padding and so forth, and it fails to account for different qualities of iron with correspondingly different prices used in different armors), but every problem it has is one that the current system also has, so it should produce results that, while not perfect, are at least an improvement.

I certainly wouldn't use the modern price for the metals (I am fairly sure that raw metals are cheaper now than they were a thousand years ago even in comparison to other goods, so treating their current price as their historical price in GURPSBUCKS would be a poor approximation), but it should be possible to get workable values.

[1] Listed in the book.

[2] Neither of which vary by type of item or amount of labor needed.

EDIT: Low Tech: Daily Life & Economics p.22 suggests $6.90 for a pound for iron, which (assuming a fourfold multiplication for bronze) comes very close to a nice, round, $20 per pound increase for bronze rather than iron.

EDITEDIT: I see Varyon proceeds me in noting the "official" price of iron, and the values he calculates for different types of armors seem not implausible and (perhaps more importantly) quite gamable.