Commodity Market prices
This site contains commodity prices for Beef, Pork, Hogs, Corn, Wheat, Soybeans, Dairy, as well as many non-agricultural commodities. This website from TheFinancials.com charges a fee for current and historical price data.

Crop Values: 2003 Summary
This site contains historic USDA data on price per unit and value of production of all crops for 2001, 2002, 2003.

Organic Wholesale Prices
This site contains historic USDA wholesale data of fruits and vegetables on downloadable Excel tables.