Is there any way to find out or determine the weight/worth ratio of certain items in D&D? For instance, how much would 3 lbs of unrefined iron (ore) be worth? Or how heavy would 50 GP's worth of Mithral be? Etc.
The Player's Handbook lists some commodities on Table 7–3: Trade Goods (112), including iron at 2 sp per lb. (therefore 6 sp for 3 lbs., although this table doesn't mention whether this is iron ore or iron in another form, iron apparently being a rather complicated substance and available in many forms).
The Dungeon Master's Guide lists an item made of mithral as costing an additional 500 gp per lb. (284), so probably mithral can either be purchased for 50 gp per 1/10 lb. or be sold for 50 gp per 1/5 lb. (because Dungeons and Dragons 3.X economics is weird that way).
The Arms and Equipment Guide also has a list of trade goods and corresponding values (40).
Underdark (for use with the Forgotten Realms campaign setting) on the lower darklands village Dupapn, Waters of Deep Hunger, describes the Mithral Pit, one of the village's important sites, partly as follows:
Anyone with the means to empty out the wells and the will to challenge a village full of [foes] would find a total of 1d4 × 5,000 gp each in gold ore, mithral ore, and platinum ore. The rocks are heavy, though, weighing about 1 pound per gp (gold), 1 pound per 10 gp (platinum), or 1 pound per 50 gp (mithral). (146)
A DM wanting to make his PCs' lives miserable (well, PCs without access to a portable hole, anyway) could extrapolate from these figures.