The minimum recommended deposit amount to trade with FBS is $5, but this may vary for different payment systems. Some might require more or less amount. For example:
Neteller: $10
Skrill: $12
You can always check the minimum requirement for the payment system directly on the deposit page in your Pesonal Area:

For a smoother trading experience, we recommend depositing at least $100. Use the Trading Calculator to determine the required amount to open an account order. Find detailed instructions on using the Trading Calculator in this article.