If you slowly add tuna to your diet it will not be a problem as your kidneys will adjust to remove the mercury (from what I understand). Don't simply start eating tuna as a cheap protein, as I said above beans and lentils are a great source of protein. As this is about budget meals, if you add your can of tuna to a few either boiled or baked potatoes that have been mashed and make your own fish cakes - this spreads the cost out and you can get a few meals out of one can. (add some frozen vegetables, spices to the mix and chill before frying or grilling).
I made a great spinach and prawn curry yesterday which was very nutritious and cost very little.
Frozen spinach (few cubes)
Tin of diced tomatoes (was in a rush but could have used fresh which would have been cheaper by the KG)
Frozen cooking prawns
couple of onions, garlic, ginger (which can be kept in the freezer and grated) fresh chili (again frozen)
Cumin, Coriander, Turmeric powder (I would say a tsp of decent curry powder would work)
I was in a rush so, tipped the spinach into a bowl with the tomatoes and put in microwave to both defrost the spinach and heat the tomatoes.
Heat up a bit of oil and added garlic, ginger, chili, fried for a few minutes and then added the cumin and chili powder (replace this for the curry powder if using), fried them for a minute then added onions, cooked those for a few minutes, added coriander and turmeric and then added the defrosted spinach and tomoto and the frozen prawns.
Served that with stir fried fried that I had cooked in a batch earlier and had frozen in portions.
If your looking to bulk this up further or cut the costs - skip the prawns and add some boiled potatoes to the mix (Aloo Palak).
If your have the time try making your own breads - the ingredients used are really inexpensive (so if you get it wrong a few times it doesn't matter).
I made this last week - half bag of floor, water, yeast, olive oil, salt and sugar, garlic, dried oregano and chili flakes (couple of $ for a full loaf)
A quick goto lunch (again from the freezer) Calamari salad, Half bag of frozen calamari rings - boiled and then run under cold water. (this was probably under 2$)
Salad and a dressing of fish sauce, lime, sugar, salt, chili and garlic
A lot of my meals are posted
here.
I honestly think its worth spending a few bucks on getting some basic ingredients sorted out - spices etc as they can turn something bland into something very tasty for nearly no added cost.