Bang for buck.. The most costs effective method would be to take a protein supplement. As FF says, go for an Isolate as this is quickly absorbed.
Diary products such as Eggs, Cottage cheese etc. contain casein which releases protein much slower than isolate. These are great for eveing snacks or to mainatin a regular ingestion of protein. I tend to mix both so i get a good supply.. I have a snack after the gym with chicken or tuna and cottage cheese, for example.
I buy my lunch at the begining of the week and take in with me. I train lunch times though... I tend to purchase 3 tubs of cottage cheese, 2 tins of tuna, 4 chicken breasts which I cook, 2 tins of mixed salad beans, toms and mixed seeds to sprinkle. It's a good balance of proteins and fibre and isn't that expensive either. I think i spend around £20 for the week. I also buy a dozen or so eggs for a morning snack. Roughly a few hours before hitting the gym. This ensures that the casein is absorbing through my workout.
Everyone is different and i'm no expert but i find this provides results. See what works for you.