The process seems very basic, but theres alot of room for error without proper equipment.
First make a clay model, then cast it in a sturdy material such as plaster,resin, or silicone. One part molds are the easiest to work with, but you can make 2 part molds.
One part molds will lower the risk of airbubbles greatly, but a 2 part mold will allow you to make more 3D products (so if you want 3D balls, you usually need a 2 part mold. With a 2 part mold your chance of airbubbles are extremely high without a vacuum chamber. Vacuum chambers are VERY expensive though.
All together one model could cost you around $100 to make the mold for, and then each product could cost anywhere from $30-$$70 to make (no those are not retail sales prices but cost prices)
Hense why costs of silicone packers are always a but high no matter how basic they are.
For silicone, dragon skin is a good one. Its often used to make lifelike babydolls.