There is a HUGE difference in a kid who wants to put in extra work and a kid who is paying his current coach to get that work in. It's slippery slope, and a conflict of interest in my opinion, and no one in the world could be impartial. It's not the amount of money, it is that Kid X is paying the coach to see him more. The coach tweaks things the way HE wants to see them, which isn't always what the paid professionals are teaching, it's unique to this coach. Kid X then has an advantage over Kid Y who didn't pay the coach. Kid X gets the nod to go in, or Kid X gets an extra chance because the coach has seen more of what Kid X is capable of rather than Kid Y. Then again, maybe Kid Y doesn't have the money for extra lessons with the coach, guess he misses his chance to improve, and Lord help us if the HS coach doesn't actually know squat. This is what I am saying, slippery slope.
Throwing hundreds and or thousands of dollars at the coach over the course of the season in "private" lessons is what is done in 9u-12u. This is what I meant by it being a Little League BS punk move. I've seen in a LOT of times, and I always thought it was wrong and never paid. It isn't the stud that gets time taken away, it's the other fringe kid who didn't pay.
Holding a kid back after practice to work on some tips is great, especially if you are asking him to do something in a specific way. Asking a kid to pay for these extra tid bits of info is ethically wrong in my book. Should kids be seeking out catching instructors, pitching instructors, hitting instructors....etc and working outside of practice YES! But those are supposedly paid professionals, the high school teacher coach spends WAY more time on the field that he gets paid for but he is paid by the school, he should not get kick backs from individual kids.