It's important to understand the distinction between "price" and "cost." The price of something represents the amount of money you must pay to buy it. The cost of something is the value of it when you compare it to alternative uses of the capital and labor used to produce it.
U.S. agricultural products are "cheap" because U.S. farmers are heavily subsidized. When you factor in the cost of a $150 billion Federal farm bill, your loaf of bread costs a heck of a lot more than the $1.99 you pay at the supermarket.
Agricultural subsidies are rather small and often go towards preventing growing crops. So I would say that they RAISE the price more than they artificially lower it.
American agricultural efficiency and productivity are higher than those of anywhere else in the world Australia and Canada possibly excepted.
What nation produces crops at a lower price (or cost) than ours?