Perhaps I could lend some additional perspective about Ebonite. From a material perspective, Ebonite is a brand of hard, vulcanized rubber invented by Goodyear a little over a century ago. Since then, Ebonite has found many uses, mostly by pen and musical instrument makers. Our friends at Paladin were the first to explore Ebonite as a handle material back in, I believe, 2014. We jumped on the bandwagon roughly 15 months ago after being commissioned to create a limited edition brush for a small group we belong to. Since then, we have periodically created a limited number of Ebonite brushes on both custom/bespoke and ReadyMade bases. (Other makers have subsequently joined the fray, as it were.)
At ground, Ebonite is a combination of rubber, sulfur, and linseed oil. As such, it's a natural material. Ebonite powder is produced in India, China, and Germany; but to our knowledge is only available in usable--for shaving brushes--rod form from one manufacturer located in southern Germany.
Ebonite is a relatively soft material--as opposed to the more commonly available cast polyester and acrylic plastics--so Ebonite brushes are more forgiving when dropped on a hard surface. Ebonite can also discolor over time, especially if not wiped clean after use. Ebonite, however--like badger hair--dislikes excessive heat and can discolor if repeatedly subjected to temperatures warmer than one's hand might tolerate for more than 10-15 seconds. (For that reason, we suggest that shaving brushes be kept from direct sunlight--although our tests, in alignment with those of the manufacturer and others, suggest that Ebonite itself isn't inordinately sensitive to it.)
From a "machining" standpoint, Ebonite turns easily on a normal wood lathe using ordinary--but very sharp--woodturning tools, its turning characteristics being closer to wood than to many other materials commonly employed in shaving brush manufacture. For that reason, those who employ CNC lathes can face larger finishing challenges than the skilled maker using traditional tools. And while a fair amount of skill is required of the maker to produce intricate patterns, like the brush shown below, Ebonite is rewarding to the careful worker who possesses the skill and takes his time to make good, clean cuts. (Interestingly, the brush below includes a miniature "portrait" of the maker centered in the handle's lower quadrant. Yes, Ebonite can be polishes that brightly!)
The downside is that Ebonite is very--one might even say "fiendishly" expensive--especially in diameters sufficiently large to create the handles needed to accommodate 26-30mm knots. Therefore the smaller maker (and we include ourselves in that category) is constrained to accept whatever stock the manufacturer has on hand; that because the minimum order quantities for specific color patterns are often prohibitively expensive, especially when multiple color patterns are desired.
So why are Ebonite brushes so expensive vs. one with (say) a polyester handle? At least two reasons: the main reason is the time required to create an Ebonite handle is, at least for me, roughly twice that needed to turn an equivalent polyester or acrylic (time IS money, after all!) and second, as described above, is the sunk cost of material.