Kids should not get paid any money for good grades.
A kid should do their best to get good grades whether or not they get paid.
Although you should not be mad at a kid for not getting good grades they should not be forced to get good grades either and should not be paid money for it.
However if your kid gets good grades you can award them by taking them out to eat or to an outing etc.
Tell the kid that if they work hard and get good grades then you will take them to their favorite place to eat or to some event they like to go to.
But payment in the form of money should not be given.