Bonus Bets are bets that do not use the bettors actual money and instead are free bets that the Sportsbook has given the bettor.
A bet placed with a Bonus Bet will pay out the winnings minus the Bonus Bet stake. For example, if you placed a $20 Bonus Bet at odds of $10 your winnings would be $180. This is calculated by multiplying the stake by the odds ($20x $10 = $200) and then subtracting the stake of the bet ($200 $20 = $180).
