Working on a plan and working hard in gym and is more of connected to food and how much calories you take in. Sometime buying all supplements can shrink your budget. Given that, can i substitute protein shake for fish or chicken, for every meal? If not protein shake, then what can i do to make food replacement (protein & carb) cheap?
Assuming i take the the said amount. Does it has any pros and cons? How does we calculate "1g of protein per pound of body weight" if we substitute.
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.
I think the mistake you are making in your question is that you assume whey protein is not food. Whey protein is food just like anything else, it's usually referred to as a supplement because it has conveniently great macros for bodybuilding (if we take this as a general example 20g protein per 100 calories in a scoop).
You can substitute 2 scoops (40g protein) with anything else i.e. chicken, fish, etc but it's not so convenient.
If you weighed 150lbs, you'd need to eat 150g of protein a day to eat at 1g per lb of bodyweight, it doesn't make a difference if you get that protein from whey powder or something else (but whey powder will make your life easier).
