You can get pretty cheap protein powder here, I’d suggest buying it at CostCo if you have access to one.
Peanut butter was my go-to bulking agent food because of the combination of protein and being incredibly calorie-dense.
Milk and eggs are also cheap sources of protein, and pasta, potatoes, and rice are all super-cheap and good ways to make sure that you’re staying at a caloric surplus (as well as making sure you absorb all them good vitamins and such).