I just bring in leftovers from home, and nuke them in the office microwave. Even cheaper.
On road-trips, I often just bring a cooler with cold cuts, yoghurt, and drinks. The fast food in highway rest areas are generally high-price and low-quality.
Best way to go even if you have to buy some inflated $5 bags of ice now and then to keep it cold . . .
As an added bonus, you don't have to deal with lines and nimrods in the fast food joints.