I've only collected royalties from this for a couple of years. It took me a while to learn that Access Copyright even existed.
The short answer is that I don't know the answer to that, but I think Ms. Brooks' answer would be the same. I have received a few thousand dollars from them, but I don't really know the number. I'm sort of focusing my message, instead of on the exact dollars, on the morality of it, on the notion of why this system expects me, the guy at the bottom end of the food chain in publishing....
Aren't we asking professors and university administrators to contribute 10% of their income to the greater good of the public education system? When MPs give up part of their income to help finance the deficit in schools, come back and ask me to donate free publishing.