Yes, we pay for grades. I look at it this way. My DS is 15 and DD is 13. If they spend a lot of time working on their school work, this leaves them little time to earn extra money by doing odd jobs and babysitting. DD has had to turn down several babysitting jobs due to school.
Therefore, I feel that as long as they are working hard on their grades, they should be compensated. We do $20 for A and $15 for B. Nothing for C and below. Although, we are discussing the "payback" for lesser grades.