If I deposit $100 or $120 worth of bitcoins per month from here, how much will it benefit me? Is it right for me to invest more $ in Bitcoin than this? And should I invest the whole money in Bitcoin?
Invest as much as you can afford to lose. How much Bitcoin you will be able to accumulate depends on the source of income. Whether you want to deposit $100 or $120 in bitcoins per month is entirely up to you as you should invest then regardless of income and expenses. If you invest all the money you earn from working on the farm in Bitcoin, you may find that you have to sell your investment before you reach your specific goals when you have family needs.
So you invest in Bitcoin in such a way that you don't have to sell your investment when your family needs money. Even set a specific investment goal and keep depositing bitcoins regularly until you reach that goal.