Not necessarily John. The Fox one, about the same price, has a stainless thread, which according to pundits, is more expensive than brass.
I too have some John Roberts back rest rod grippy things, some luminous too. They're ok, but I would like to keep unscrewing them.
I would have thought that the total cost of materials for a plastic and brass rod rest would be in the region of - - - - - - - 20 - 30p max