No, I won't pay my children money for good grades. I expect good grades because that's what they're capable of. When the girls perform to the best of their capabilities, they earn privileges. When they excel, they earn privileges that are out of the ordinary. They also get praise and compliments for a job well done, and they love the feeling of accomplishment they get from performing well.
I don't expect straight A's. I expect them to perform to the best of their capabilities. But I happen to know that my eldest daughter is honors material. Those are just the sort of grades she's used to pulling without much effort into it. If she puts in the effort, it's high honors.
We're big on natural consequences. But simply getting poor grades isn't enough of a natural consequence. There ARE consequences at home too. Not groundings, but loss of privileges. Not just "not earning" privileges but losing current privileges. Additionally, we increase study time and homework time, reducing social time and free time.
Those things are their currency right now. Would they like money? Sure. But it won't motivate them the same way privileges do. Social activities with friends and family are much more important to them. Besides, whatever they buy with their own money we would probably buy for them down the road anyway. It's just a different way of earning.
They do get money as gifts at birthdays, holidays, and special occasions plus their grandparents and great-grandparents love to hand them $5 for no reason at all, so they're not being deprived of the chance to figure out how to use it. We involve them in helping us with grocery shopping and clothes shopping and other types of shopping. We teach them about budgeting. Sometimes we bargain for a video game in regard to a large household chore.
So when it comes to school, money stays out of the equation.