Betfair operate a layered commission structure whereby the more you bet, the lower your rate will be. Commission is charged on your net winnings from any single market.
You do not pay any commission on losing bets.
Their standard commission rate is 5%, but you will be able to decrease that depending on the volume of bets you have matched.
Commission amounts can easily be offset by the increased odds you get betting on a betting exchange, opposed to betting with a traditional bookmaker
To open an account with Betfair click here