If you put in just 50 dollars per month, every month and can do something as simple as 25% per year, which is very doable in bitcoin world, you will have 10+ million dollars. Yeap, not wrong, you will have 10+ million dollars in 40 years...
I have to correct you a bit, as the result will be exactly 2 times more than what you wrote - 20 million dollars. This is exactly the amount that will be obtained, taking into account the compound interest, for which a profit of 25% APY will be accrued. But the problem is that there is no guaranteed percentage of 25% APY in the crypt, and you cannot rule out the possibility that you can lose all your money in one day.