A poster named mydisease has posted and updated a tech valuation formula at CFC, and now another poster named The Grey Fox has written a utility to do the math for you.
Unfortunately, the formula (and the ultility) only calculate "research cost" and not "trade value." The value of any given tech in trade is not reflective solely of its research cost -- wonders enabled, extra abilities conferred, new units, "tech tree chokepoints," etc. seem to affect trade value.
Also, resources (luxury or strategic) vary widely depending on the buying civ'e empire (strength, number of cities, population, position in tech tree, city development, etc.) Twenty turns of silk may be worth 20 gold to one civ and 20 gold
per turn to another.
Catt