Marty raises a number of interesting points.

I got a big piece of leather from which to make saddles as I learn to recover them myself. It cost me $130 plus shipping &c. It varies in thickness, and there pretty much countless ways I could cut it up into a maximum of about a dozen saddle blanks. Presumably some parts of it are better for making saddles than other parts, and to make the best saddles one would have to know exactly where and how to cut each piece. The thickest pieces are not necessarily the best pieces. Some pieces may be too thick, in which case they may be shaved down to the ideal thickness. The people at Brooks presumably have the expertise to do this, and they charge you for it (as they should). Whether the people making all the "off-brand" saddles have this expertise, I can't guess. As for me, well, I'm learning.