Why should you eat every day meat (fish/chicken/cow etc)
Try to have two vegetarian day's a week. Only some pasta, tofu, beans, low-fat cheese (quark/ricotta).
The alternative for canned tuna is frozen tuna.
Over here it costs about 1,70 for 250 grams and it contains two pieces. You can make you own hamburgers with it, exactly the right size for a sandwich.
And about budget,
quark costs only a 49 cents and contains over 50 grams of proteine. Perfect food for the evening,
very low (no) fat, low carbs, plenty of proteine. You only have to get used to the taste, familiar to yoghurt.