A customer will always get the best odds available at the time of the bet being matched; If the best odds available are offered at a smaller stake than the customer's desired size of stake, while the second best odds are available at a larger stake.
Betfair has a standard commission rate of 5%. That applies to almost every sport, in every country, around the globe & except Australia. Due to the differing fees charged by the NRL and the various racing commissions, Betfairs commission rate in Australia is variable.