For large accounts, transaction cost seems less an issue than small accounts. As we know, transaction cost does have an impact on overall return. An algo which conforms to the Q's contest requirement has a lower return because of its lower risk profile. Thus, transaction cost can further reduce the returns to a major degree. Such an algo which is profitable with a large account may lost it edge or even becomes a loser after transaction cost if trading with a small account.
Currently, Q has per share and per trade commission schemes to model transaction cost. Here is an interest find that points out that the real world transaction cost is around 0.06% ~ 0.12%, much lower than the academic estimates of 0.6%. Is it possible to add a percent commission scheme in Q ?