Exchanges take a percentage of your net winnings on each market. Losing markets pay no commission. The rate depends on the exchange and on your account, so enter your own.
Worked example
£10 at 5.0 wins £40 gross. At 5% commission you pay £2 and keep £38, the same as backing at 4.8 with no commission. That effective price is what you are really getting.
Why it matters for trading
A scalper making many small greens pays commission on each winning market, so a 1-tick edge at short prices can disappear once commission is included. Check your net result, not the gross one.
Questions
Do I pay commission on a hedged trade?
Only on the net profit in that market. If you green up for £2.50, commission is charged on £2.50, not on the stakes.