The Los Angeles Times published a nifty article tying in the current economic situation, the horrendous farm subsidies ("for the last 60 years or so, the government has subsidized the production of commodity crops -- corn, wheat, rice and soybeans -- that are ingredients in many high-calorie foods... to receive the subsidies, farmers must refrain from growing any fruits and vegetables,") and nutrition.
The article also highlights a study published last year in the Journal of the American Dietetic Association, which tracked the prices of 372 foods and beverages sold in the Seattle area for a two year period (2004 - 2006.)
The conclusion? "The average price increase was 7.9%... [but] foods most dense in calories had dropped by an average of 1.8%, [while] prices of the lowest-calorie foods had gone up by an average of 19.5%."
As discouraging as that may seem, here is my by-no-means-exhaustive list of affordable and nutritious foods you can rely on (whenever applicable, buy generic):
Plain yogurt (non-fat or low-fat)
Plain quick-cooking oats
Whole wheat bread
Natural peanut butter
Brown rice (cook in large batches and refrigerate)
Ground flaxseed (a two pounds bag costs between $4 and $5 and will last you months)
Canned beans (I suppose dry beans are the true money saver, but canned beans are inexpensive and a wonderful source of lean protein)
Potatoes (the key is to keep the skin on and cook them with little added fat)
Garlic (an inexpensive way to add flavor)
Canned tuna (ideally chunk light and packed in water, to preserve the Omega 3's and slightly cut down on mercury levels)