Just to clarify, resin for miniatures is a synthetic resin which is a plastic. The cost differences between plastic (injection molded plastic) and resin (cast plastic) is cost to manufacture. Resin models are made in inexpensive low durability molds which limits the amount of models that can be manufactured. They generally require secondary operations to reduce bubbles and to remove parts with lots of detail. Injection molded miniatures require an expensive mold tool, but parts can be made into 100k+ volumes. The injection molded plastic parts typically have less detail because they must easily clear the mold tool without operator intervention. Metal miniatures, like resin, are cast. I've never understood why they are less expensive than resin since the manufacturing process is so similar, but I assume it has to do with part volume per mold.