My reply was based on the assumption that one is getting fair price execution from whatever broker one is dealing with. If there's PFOF or some other reason for a sub par fill then that's a different story. 500 shares round trip at IBKR costs $5, as long fills as fills are 200 shares or more (the $1 minimum ticket). At tastyworks, it's a flat fee of $5 to open with no closing commission. Clearly, one's trade size determines which one is cheaper to trade with.