The creator would make more money if he sold them cheaper. They would sell in much larger quantities.
I think Kinoko Juice dolls are made in small batches since they are high quality dolls that are hard to make. The dolls are made out of resin which is also very toxic and difficult to work with. The resin has to be hand mixed and hand-poured into molds. When the parts are removed they have to be cleaned up by hand and the seam lines need to be scraped and sanded. The molds of the dolls also need to be remade after a relatively small number of uses, which would definitely bump the price up a bit. The materials of clay and polyurethane resin cost more than the vinyl and injection type molding used to make regular dolls, so, they cost more per doll to make. There is also a lot more manual labor involved, instead of everything being done by machine, which also makes them very expensive. If there is a faceup in the making of the doll, the face needs to be sanded, needs 2-3 coats of a toxic seal, then is hand-painted by an artist, sealed again. Then gloss accents are added and real lashes attached. Then eyes are placed in the head with putty. It’s mostly labor intensive.