If your salary is $400 monthly, you can be investing like $20 on bitcoin which is 5% of your monthly income .
He didn't say that his monthly salary is $400, but instead that he can invest $300-$400 per month.

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@OP Despite bitcoin price hovering around its all time high, don't think that you are too late to the party so you turn to altcoins instead, mistake that many of us when we got in the contact with bitcoin for the first time. Just be aware of the fact that you bitcoin won't make you rish by investing few thoussands of dollars, and that you may have to wait for years to get significant profit.