Yuval,
This is a great question. I believe we (all of us) can get some answers with a little research. But I do not think we can find all of the answers—with me being the last to learn everything. Much of this you already know.
First, there is some limit to how far they can extend beyond the BID/ASK spread on your order. They are required to fill your order within the BID/ASK spread (NBBO)—for the amount on the order book at the time of the order. As you know NBBO stands for the National Best Bid and Offer. I think they meet this obligation (as far as I know). Of course, if the book has only 100 orders at the prevailing ASK (for a buy order) then only the first 100 stock are bought at the prevailing ASK under the regulations. Your next 100 (or whatever number) can then be filled at the new ASK according to the regulations.
As far as I can tell we can do a little better (sometimes) in the following ways (probably not an exhaustive list).
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Some brokers have access to (and give you access to) dark pools and some part of your order can be filled between the bid and the ask.
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Fidelity (as an example) has market makers who will fill at the prevailing BID or ASK beyond what is required by NBBO (will fill at the BID or the ASK for more than the number stocks on the book at that that price). They are nimble enough with enough access to dark pools and multiple exchanges (with computer assistance) that they can still profit by doing this.
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For VWAP orders the market maker will try to “Make” liquidity. When they do you get a savings and they keep the rebate for being a “Maker” and not a “Taker” (of liquidity). Many exchanges offer a rebate (which is usually not passed onto the trader) for “making liquidity.”
There was publication of results of price improvement in the past (IB did not participate). Charles Schwab has done well regarding price improvement historically (as has Fidelity).
For limit orders: try routing through NASD. This is anecdotal so try it is my only recommendation. See this link for a limited (and old) reference of the NASD routing of limit orders for retail investors: https://www.finra.org/rules-guidance/notices/94-58. While old it does show that NASD has made efforts to protect the retail investor.
Finally, I think IB charges a commission and makes a profit from your trades in other ways too. IMHO, you have to get the high volume discounts, get some of the rebates (“Maker” of liquity) and be savvy regarding routing and ALGOs for IB to work for you.
I think SUPirate1081 is savvy and makes IB work for him: he tries to be a “Maker” and collects a rebate for doing this when possible. Maybe he will comment again (updating and correcting some of my impressions).
Would love any corrections or additional information. For now I like Charles Schwab for simple orders.
It goes without saying that whatever I think I know now could change with the commissions war.
As far as how brokers can profit through payment for routing, that takes no imagination. They just pocket the payment. How do the people who pay for routing profit? It would take a lot of imagination to figure out how they wouldn’t if they have access to enough dark pools and large block orders that are to be filled over an extended period.
-Jim