It seems that Polyurethane is a good material for making models. It can exist in different densities/hardness and cost is around 6$/kg. This means that the cost of the material for a milled model would be peanuts...lets say $1. What would be the cost of a milling machine ? 90,000 $ for a very expensive 5X machine for making models, and that would produce 30,000 models (in 3 years),then we have a machine cost of 3$ per model. Tools ? 1$ more. If my maths are good, then we could mill a model for the cost of $5. Hmm. this is lower than stone !