What do you think?
(Actually, it's a completely ambiguous question, because value is (a) dependent on fluctuating market forces and (b) dependent on the form of the material. E.g., carbon can either be extremely valuable (diamond) or inexpensive (graphite). Even within these categories, price can vary substantially. The value of diamond depends on its color, purity, crystallographic quality, and quality of its cut (for jewelry). Highly exfoliated forms of graphite are much more expensive per mass than the most unrefined coals. And so on....)