Betfair charges Commission only on your net winnings on a market. You do not pay commission on losing bets. How much Commision do I pay? This is how it is calculated: Commission = Net Winnings x Market Base Rate x (100%-Discount Rate).
Trading the Betfair markets is most definitely a genuine earning opportunity and for those who are determined and committed enough, it certainly can and does replace a full time job. And then some.