how does interactivebrokers compare to this in terms of cost? are there any hidden costs retail investors should be aware of?
You must pay at least $10 in commissions each month or they charge you the difference. Depending on how much you trade this might be a deal breaker. There is also a minimum initial deposit.
Virtualbrokers is another option with more choice of fee structure. I don't think they have a minimum monthly fee but they charge you for data.