Not the cheapest ever but I have been playing with variations on tuna salad/melt.
The other day I did can of tuna, small onion, bunch of chopped baby spinach mixed together with a few tablespoons of mayo and some salt and pepper. Add a little tobasco sauce or whatever to give some more flavor. This gave me enough tuna salad to spread on to 2 slices of bread as well as make a sandwich for the next day's lunch. Throw the 2 slices into the oven until they are hot and then toss some cheese on top and place back in the oven until it melts.
maybe $3 worth of stuff in here for a dinner and lunch.
If I had them on hand, I would have chopped some tomatos....but otherwise it was pretty good.