I think that when you get to $25 an A that you are PAYING kids--or trying to anyway. Having a reasonable incentive/reward for a job well done isn't necessarily going to get a struggling kid on the honor roll, but it may help them pay a bit more attention.
As kids, my brothers and I were given $1 an A or $20 for straight As. I was a nerd and a 4.0 student pretty much all the time. On the other hand, my older brother struggled all through school despite being offered obscene amounts of money to even get straight Bs. Turns out he has a minor case of dyslexia that the Marine Corps later diagnosed.
Even though I got good grades on my own, it really made me feel like my parents acknowledged all the hard work I had done...mostly unsupervised. In fact, I continued to extort the reward all the way through college. Hey, I was on a full ride scholarship...it isn't like they had to pay for my tuition...lol.
I have to add...we NEVER got an allowance for doing our chores around the house. My parents took care of all of our basic needs, plus some of the wants. They always provided guidance for when we started babysitting about reinforcing the importance of saving for a rainy day.
You really just have to decide what it best for your family and adapt with the situation. It sounds to me like you are doing a great job.