I think that is to be expected in a free market...farms have been more productive (herbicides, pesticides, machinery, bio-engineering, etc.) and global markets have been tapped to increase the supply of wheat. While demand has also grown, supply has simply grown more, thereby bringing down the price.
How can farmers survive? Last summer some friends invited me to a "pizza farm". I think it was a normal farm, but it had a few outdoor pizza ovens and they would make pizzas with ingredients grown right there. The pizzas were expensive (around $25/pizza) but they were quite good, and the place was pretty packed even though the place did not advertise and didn't have any signs leading to the place. People brought their own beverages, lawn chairs, and ate outside on the large lawn. When I went, the farm only did the pizza sales one day a week during the summer. I thought it was a really ingenious way for the farm to survive and probably make some pretty good $$ by catering to people who wanted an alternative to the typical restaurant experience.