In my opinion X touched on a detail which makes this subject incredibly difficult. It is difficult to assess how much success is a result of the trainer and how much is a resultant of the fighter's own natural ability. Furthermore, is it correct for a trainer to accept credit for a fighters success in situations where the fighter may have had numerous amateur coaches who approached their job as diligently as the professional coach. In modern boxing there are few cases where a trainer truely builds a fighter from the ground up. Many fighters are elevated to national status, then they are promoted to an International coach...in the amateurs! So that really does question the coaches effect.

I believe one method is to survey the "mediocre" fighters that the coach trained. Eddie Futch took on Silver Olympic Medallist Wayne McCullough, who although a good amateur was not a sensational talent. Futch was an inspiration to him and he did extremely well for himself.
Cus D'amato took on Buster Mathis Sr. and had the ability to strip pounds of fat of his frame and turn him into a genuine contender.
The Pettronelli brothers took a rough and ready amateur and turned him into "Marvelous" Marvin Hagler. They also took the even rougher Steve Collins and polished him enough to win two world titles.

So, in summation I feel the value of a trainer is ascertained in his relationship with the journeyman or the mediocre fighter. I believe a coach is at his most valuable when he can take a man in need and grant him the ability to make a decent living with the skills he teachs him.