I am the king of cheap!
For protein I would look into eggs, chicken breast (1.99 per pound), plain non fat Greek yogurt (1.99/lb) and even tilapia (2.50 per pound). I would not suggest tuna as it is more expensive per pound when you do the math out.
For carbs, both for price and conveince, I would suggest rice, sweet potatoes, rice cakes, oatmeal, cream of wheat.
For fats, oils, peanut butter and nuts.
Here are some sample meals that I prefer for work and I take on a weekly basis.
Everything I suggested could be made at once and you only need to prep once per week. For the sweet potatoes, buy a rice cooker with a steaming basket. You could cook both the rice and sweet potatoes at once or even steam veggies while the rice is cooking. Chicken, buy a slow cooker and you can cook a lot in a period of time which you do not need to stay in the chicken.
If your body is burnt out from plain foods, just lean how to use spices and seasonings. Learning how to enhance the flavor of plain, boring foods can go a long way. "Healthy" foods do not need to be boring. Also, marinade you meat overnight and add some sauces if needed.