The amount of usd needed for trading your trade depends on your purpose as well as depending on the amount or type of coin you trade such as bitcoin, you will need large amount of usd also if you deal with ADA or ripple less usd
Yeah trading bitcoin its need a good amount, thats why a lot people try & start trade with a low amount money into a Altcoin with a cheaper price & young project. Its really a good choice for started.