We tried to communicate to our children, even when they were younger, that doing their best work, performing honestly and responsibly in school (not being tardy, turning in work on time), respecting the teacher and having a reputation as a trustworthy hardworking student had its own rewards.
The rewards weren't in the form of money for A's, they came in less tangible but important ways. For example, establishing a history of getting good grades meant that if a special opportunity arose (a chance to attend a once-in-a-lifetime event that would mean missing a day of school, for example), the child could take advantage of that. A child whose grades were not stellar might not be able to afford a missed day.
A child who was regarded as trustworthy and of good character and whose grades are excellent might be selected for a special honor or privilege. Some schools have standards for taking part in theater performances, for example (students with poor grades can't afford the time for rehearsals), or for joining sports teams (poor grades mean you can't try out for the team), or for honors like introducing the celebrity speaker at a special assembly.
So I think it's important to instill in children that good report cards don't earn money, they earn something much more important: a good reputation, a positive impression, and the trust and respect of their teachers. All of those things are taken into account when applying for leadership positions (running for student council or an officer of a club), honors (such as being allowed to represent the school at a conference or event), attending the college of their choice, and even participating in extra-curricular activities (applying to be a camp counselor or attending space camp, for example).
On a few limited occasions over the course of our kids' school years, we let them miss a day of school (Uncle Joe had tickets to the game of the season, for example, or we could extend our vacation by one day) and because their grades were excellent, we were going to let them go, or spend that one last day at the beach. Had they demonstrated mediocre work and poor grades due to laziness or apathy, they would have had to miss the game opportunity, or the extra vacation day, and go to school as usual.
The other problem for paying money for A's exists when a child does his or her best work, but due to illness, disability,or some difficulty, an A is just not possible. But for that child, getting a C or even a D is a huge deal. My daughter has severe medical problems, and attended school online. My son has no such issues and we expected (and he received) excellent grades. However, for our daughter, simply participating in the class, working through the pain, and staying in school even when she wanted to give up was a triumph. Her grades were pretty awful, but we celebrated her efforts. Lying in bed day after day, taking pain meds, having surgeries, but still trying to complete her assignments were monumental achievements for her.
I think that instilling the importance of reputation and character is something a lot of parents miss these days. We don't pay for grades, but we certainly recognize the pattern of hard work and ethics in many other ways. And the world does too. Earning excellent grades (or doing their best work to the best of their ability) means that a child has demonstrated good work ethics, completed tasks, fulfilled requirements and performed well, and that sets a child up for success in life and the chance to take advantage of opportunities like college, the career of their choice, and other benefits. We need to teach our kids the value of hard work.