You asked about how to eat tuna, i eat it several ways.
In a sandwich
Right out of the can plain, or with salt and pepper, or with something like steak spice sprinkled on it, or with a tablespoon or 2 with mayo
Tuna melts: mix with a tablespoon or 2 of mayo, put on bread, put 2 thin slices of cheese on top and cook in oven until cheese is melted
Go for the no name tuna if your on a tight budget. Here the popular brands cost 2.50 a can but the no name stuff only costs 1.00. It has a couple less grams of protein per can but its worth the savings
You can also try mixing it into a omelet. Or whatever get creative
Agreeing with the people above nuts are good. Peanuts are particularly cheap. I buy them in the bulk bins at my grocery store unsalted for 22 cents per 100g. Not exactly low fat but its good fat.
Always check the flyers of your local grocery stores. A good sale can turn a good source of protein that is usually too expensive into a really good deal. You can do this online these days for most stores.



