This topic can be discussed at so many levels, so here are a few of my scattered thoughts on the subject. As far as what percentage of total production the big tree makers have, I could not hazard a guess. Ther are several tree factories that turn out a lot of trees and they sell to all manner of saddle builders.
A good quality production tree can be as good or better than a halfast custom hand made tree. That having been said I think a well designed well built hand made tree is better than just about any production tree.
One function of a custom tree maker is to make trees that might not be available from mass builders, and we have the perception that the quality will be better as well. We have the same perception of the custom made saddle. Our perceptions of both the tree maker and the saddler can be faulty.
Large scale tree and saddle factories develope lines of product that can be made and marketed in mass, which gives them the edge on price. Quality of these products can often be good.
Terms used to describe saddles and trees can cunfuse as well, custom, hand made, shop made and so on. To my way of thinking a custom made saddle is one that starts with the customer and their idea of what they want, from there the saddler orders the tree and such, then builds the custom saddle. The tree used in the custom saddle can be a stock production tree, a semi-custom tree, a custom production tree, or a hand made boutique tree, as long as the specs. and quality is what the customer ordered.