your statement is correct for the everyday joe. however, what good is best commission if the order does not get executed or is filled at a sub-par price.
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.
are you or him paid by IB?