Is $50 the official point where it matters if you be a gentleman or not? Some people would think that 50 bucks is the world ... others would drop 50 bucks out of their pocket and not even notice it.
For me, I always tell the other person if they are about to hit the wrong ball UNLESS they have been trying to shark me, been a loudmouth, tried to distract me in any way, etc. Basically, if they are nice, I am nice, and the amount of money doesn't matter at all.. if you are used to playing for 10k+ a set I'm pretty sure that the magic $50 mark doesn't apply, hehe.