Quote from SizesTrader:
The fees for most pro firms are pretty cheap nowadays. It depends on how you negotiate with them. If you are paying $50 for 1000 shares, that is very cheap. Most of the pro firms out there will probably charge you around $65 - $100 per 1000 shares each way, buy or sell.
If you lose more money than your initial deposit, you will have to deposit more money. Pro firms have risk management that keep you from losing all your money, but if you do, then you have to deposit more money before you can start trading again.
Hope this helps!