Ahh, finally a subject I am a certified expert on!
Pack some instant oatmeal packets in your rollaboard. You can buy a box of 45 for about $9 at Sam's club. Heat water in the hotel's or airplane's coffemaker, pour into a cup, stir & serve. Low fat, decent protein, a little high on the sugar.
Breakfast cost = 20 cents/day.
I buy a 3 1/2 lb. can of mixed nuts and a 5 lb. bag of raisins also at Sam's. Then I mix the two into ziplock bags to make my own trail mix. I'll stuff a ziplock bag into my flight case to munch on when I don't have time or money for lunch. The nuts have protein, raisins supply the carbohydrates. The nuts are a little high on fat, but tolerable in moderation. Very filling.
Lunch cost = $1.06/day.
Beef Jerkey works well to supply protein, and is very portable. Sodium is a little high, but fat and carbohydrates are low. Once again, Sam's club can supply you beef Jerkey in bulk relatively cheaply.
Snack cost = 50 cents/day.
Sam's also has what they call "energy bars". They're over in the bread area. Unlike most "energy bars" that are just candy bars marketed as overpriced heath food, these are actually made of whole-grain bread, with raisins, nuts, dates, etc. mixed in. Not great tasting, but decent nutrition. A couple will make a small meal.
Dinner cost = 75 cents.
I usually try to eat one good hot meal with vegetables every day. This usually requires a trip to a restaurant. Fresh fruit takes up a lot of space, and usually doesn't survive beyond a day or two. You'll start to get funny looks if you pack bananas for more than a day, since the front half of the airplane will start to smell like them.
If you're down in florida on a quick turn, you can catch a couple of those lizzards that run around everywhere. Poke a stick through them, wedge the stick in the engine cowling, and roast them in the APU exhaust. If you're quick, you can start them roasting during your first walkaround, then pick them up when you go back out to check the fuel door after the airplane's been fueled. You haven't paid your dues enough if you haven't had diesel-flavored roasted gecko!