Quote from def:
Keep in mind IB doesn't pass on SEC or other pass through fees for shares. It also doesn't charge any minimum fees to scale into a large trade. Finally, after 500 shares, the fee is 1/2 penny. Sure that adds up when doing large size but that is a professional fee (and even better than most prop houses since SEC fees aren't included) and any way you slice it, it still adds up to breakeven on 1 tick for true direct access.
Many firms offering fixed rates for high volume are
a: subsidizing the large trades through the majority of trades that are sent int at small size;
b: selling order flow to cover the cost;
c: losing money.