First things first, nothing can substitute food. Protein supplements should only be taken when you know you simply can not eat that much. Protein is actually cheaper.
Right supplements can actually save you money, if approached in a smart way.
A chicken breast that has 20g of protein costs 1-2 euros depending what you are buying.
A serving of protein costs .50-1 euro, also depending on what you are buying.
That being said, you can not replace (it is not healthy) food, with only supplements. From fish you get many other nutrients. E.g. different fatty acids, like omega-3.
For example, instead of eating 2 cans of tuna, i eat 1 can + 1 serving of whey protein. That saves me a little bit of money already. If doing something like this daily, you can actually save a lot. But again, don't completely replace food, just use supplements to aid you a little bit, while still eating real foods.
I do believe in the "1 g of protein per pound of body weight", but that is only when you want to build muscle. Also, it is not important to start from the very beginning like this. Just start consistently going to the gym, eat more calories than you burn, but still eat plenty of protein and carbs.