It's pretty damn easy to find. The most obvious proof is in the fertilizer industry. The price of fertilizer should be one reasonable margin of profit above the price of your inputs--natural gas and...well, air is free, but that's another constituent of nitrogen fertilizer. But if you look at the history of the price of fertilizer over time, you'll see that it does follow the price of inputs for a while, until the price of grain goes up. It immediately follows the price of grain. We've got quotes in this report—I guess you didn't read it—from Agrium and from Yara, the self-described world's largest fertilizer company, that say “Nitrogen prices follow grain prices” and “Fertilizer prices [are] linked to grain prices”, not the costs of the inputs. They charge the maximum that the market will bear.
On June 11th, 2009. See this statement in context.