i think the minimum investment for eight heap size is eight000 usd as a result of 8 heap size is incredibly huge and really risky , even monger who has an enormous capital use eight heap for commercialism , you have got to create an enormous investment for eight heap size to be safe from call and It will depend on what pair you want to trade in and what leverage you use. In case you want to trade gold in inst-forex and gold rate is 1606, you need 1606$ capital to trade a single lot at 1:100 leverage but in case of silver all you will need is 29$ for a lot at 1:100 leverage .