Hi Ali - There is no 'bonus' as such, but there is a reward for going out first. Namely, you get the total of your opponent's unplayed letter scores added to you score, and subtracted from theirs.
So if your opponent has a 9 point rack after you go out, they must subtract 9 points from their score while you add 9 points to yours.
That makes a 'bonus', if you want to call it that, of 18 points.
Trying to be the first to go out in a game of Scrabble can be a game changer!