I like to compare my bike touring food costs to a similar activity, long distance hiking. My last long tour, 4450 miles across the US (Northern Tier) in 55 days cost $12/day in food. I hiked the AT a few years earlier, 2180 miles in 106 days, and that cost $12/day in food. A trend maybe? Both trips were faster than normal, and I finished both at nearly my starting body weight and was able to return to a job as a firefighter without noticeable loss of muscle, so I think I did well with nutrition.

A note on cheap food--I think it's a false economy. As I do more long, strenuous trips, I find myself craving salads over ice cream, and will often spend $4 on greens rather than a milk shake, for instance. Try to avoid a "white food" diet.