I do not think there is a hard-and-fast rule when it comes to how much USD you need in order to trade. At best, it is YOU who should know how much money you are willing to lose. You should do the math, and not rely on others no matter how experienced the latter are in the field of trading. Also when computing how much you will shell out, make sure there is enough left for you for your daily sustenance. Do not let your hands get empty just because you jumped into an investment. Make sure you only walk little by little. Success stories will teach you perseverance and patience along the journey, so please, do not bet your all in it- no matter how certain you are to earn.