I was going to start a thread about business travel, but found this one. Awesome. Almost everything I was going to discuss is applicable to the discussion here.
I travel for work quite frequently, usually leaving on Monday and returning Friday, spending the four nights in between away from home. I get my plane tickets, hotel, rental car, and other expenses reimbursed so those are a wash; but also I get a nominal per diem amount for meals & incidentals. Usually this is only about $35-50 per diem, depending on location, which is about enough to cover one fast food lunch, one dinner at a casual dining type place, a beer or two, and other food throughout the day (job is physically demanding, I also do strength training, and am pretty hungry as a result). But over a weeks' trip, if I come up with more frugal food options, I can make a good $200.
I always try to stay at hotels where there is free breakfast, since I get reimbursed for the hotel anyway. After filling up, grab an apple, muffin, or yogurt or anything portable as I leave breakfast, to eat as second breakfast. For lunch I usually have to end up spending $7-10 due to my schedule, but have had some success with making sandwiches if I can find a half-loaf of bread (the full loaf is more than I can eat in four days). It's easy to use up one pack each of lunch meat and cheese in that time though, which is convenient.
For dinner, I still tend to go to a casual dining type place, but I'll find the highest protein+calorie item on the menu, and similarly choose the highest calorie sides if they are included. Yesterday (I'm in fact on the road now) I found a plate at Ruby Tuesdays with 14oz of grilled chicken, mashed potatoes and beans for $10. Way more than I would have paid to cook it myself, but I filled up and got to keep most of my per diem. I'll forego desserts and additional sides, and drink only water (though beer is a weakness of mine!).
I'm the best at this when I travel alone. When traveling with colleagues, they always want to go out and spend as much as they would have spent anyway, PLUS more because they're getting per diem. Doesn't help that one of my frequent work travel destinations is Vegas!