Your cheapest option for lean protein per pound is gonna be chicken breast. If you see a sale, stock up and freeze some. Other than that, ground turkey, steak, and fish (depending on the sales) will be relatively inexpensive options for protein.
Carbs are insanely cheap - bulk rice, 3lb bags of potatoes, old fashoined oats. All very inexpensive options for quality carbs.
Fats can get pricey depending on what you go for - large containers of pb is a good/cheap option. olive oil can be found for reasonable prices and can also be used for a variety of things. nuts are expensive, but if you shop sales you can find almonds and peanuts at reasonable prices.
Go to youtube and check out the Animal videos that show the Universal athletes grocery shopping. They're given a budget and they have to shop and prep their food within the budget. I think the one video is called "huge on 100". Very informative videos if you're trying to save some money in the grocery store