I don't think this is cheap but I had an Arby's Philly Beef Supreme with mayo instead of the garlic sauce and just took off the bun and ate with a fork. There is a ton of meat there. It was 4.30 for just the sandwich. It is filling though.
I still agree bringing your lunch is the cheapest and fastest and most nutritional way. I would work on bringing left overs see how that works for you. I like a grilled chicken breast warmed up with cheese melted and asaragus. Or taco salad make the salad and take some left over taco meat from the night befor with melted cheese and walla!!! You have the old faithfuls TUNA SALAD but yuk I am sick of tuna I ate that for the first month and now I try to have it only now and then. At any rate lots of luck to you...