The My Profit and Loss page is available in the Betting Activity section of My Account and reflects your returns minus your stakes for all bets settled within the chosen date range of 7 days, 30 days, 3 months or 12 months (including any bets placed outside the date range but settled within it).
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).