There are several factors taken in account in order to tell how much a trainer makes. It is not the same how much a trainer gets from being paid than the total money is left. It depends a lot the local demand, the costs in the area, etc. It is not the same developing a business in Wellington than were i live. Demand is not the same, but balancing costs, It is cheaper to develop a business were i live. Anyways, this does not mean you are going to have half of the revenue a trainer would have in Wellington (for example).
I do not know if It is clear or not, but sure it is the reality of tons of businesses, not only equestrian ones!
If any doubts left, feel free to contact me!