Here's a spreadsheet I've worked out to address this exact question I had when trying to create my own food budget. I am able to maintain my weight at 195 lbs spending $12.51/day. It requires some of your own research. Obviously, you can't use all the same foods I've input here because you may not like those foods, can't find them, or costs a different price in your area, but it will tell you the cheapest way to eat what you need.
First, price shop. Get the best rate per serving. Get a notepad and write down the nutritional facts for some of the foods mentioned in the posts previous to this one and write down the cost. Your notes should contain the following:
Price
Serving size
Servings/container
Calories
Fat
Carbs
Protein
Use these notes to calculate price/serving and enter it under the "Price" column (B). Also enter in the rest of your info (Food name/Price/Calories/Fat/Carbs/Protein). The spreadsheet will calculate the cost per Calorie/gram of Fat/Carb/Protein in the yellow highlighted column.
Then if you look at your diet (which should be worked out using a plan I'm not going to get into on this message) and you say, "I'm lacking protein. What's the cheapest source of protein I've priced?" Highlight the yellow Protein column by clicking "N" at the top of the spreadsheet. Under the "DATA" tab, tell the spread sheet to "Sort" the list from "A->Z" (least to greatest value). Now the foods you've entered will be listed in the proper order.
Of course, you'll have to do some sifting here. If you look at my entries, it says "Craisins" are the cheapest protein I've submit. This is because there is no protein and so therefore there is $0 spent on protein. That wouldn't work. A little down from that, you have Hard Red Winter Wheat (highly recommended BTW) and Rye Berries. Those might be cheaper protein than Whey Protein supplements, but how much of these grains am I going to have to eat in order to get the protein I need? I don't know off hand, but I don't have the stomach space for it. Next is Whey Protein supplement, my real answer. Its the cheapest, most reasonable source of Protein I've input.
Message me if this is unclear or if you have problems running the spread sheet.
I can tell you from my experiences: If you can find Hard Red Winter Wheat, replace your rice with this stuff. It has almost as many carbs, more dietary fiber, and loads more Protein (even more protein than "super grain" Quinoa). On top of all of this, it is slightly cheaper than rice. Check for it in your market's bulk isle, especially if you have a Whole Foods, Wild Oats, or equivelant organic food market.
Also, like someone had suggested, find a warehouse that sells food in bulk like CostCo or Sam's Club. I buy deli-style turkey there for ~$2.59/lb whereas if I went to the deli and had this sliced for me I'd pay $4-6/lb. I can get Muscle Milk at Sam's for roughly $1.50/ea, and Protein bars <$1 with 30g Protein.
Other suggestions:
Solid White Albacore canned ~$1/can
Eggs
Oats
