At that size, IB's cost advantage is not necessarily overwhelming (except for securities options!), but they are still the cheapest. And if you want to buy 100 MSFT for 53.01, 100 MSFT for 53.05, 100 MSFT for 53.10, 100 MSFT for 53.15, and 100 MSFT for 53.17, you pay the same $5 that you pay for...