I'd give a lot for more minis of the same incredible quality of those two plastics. I still have to shake my head at the necessities of casting. Metal minis - rubber mold. Plastic minis - metal mold.
I grew up in a machine shop, the son of a tool and die maker, and I worked with injection molding machines through most of high school. The reason the molds need to be made of metal is due to the temperatures and pressures involved. In order to injection mold, you have to heat the plastic up to it's melting point and then squirt the plastic through a tiny little sprue hole at very high pressure. The pressure is due to the fact that the plastic starts cooling almost instantly after leaving the heated screw, so you have to fill the mold before the plastic gets hard enough that it starts to show stretch marks or just plain won't flow anymore.
The mold itself has sealed channels running through it that you pump an anti-freeze-like mixture through. This gives you total control over the mold's temperature (which is usually set somewhere just high enough to burn you if you touch it, but not high enough to burn you severely--this I know). Each shot takes somewhere between 5-15 seconds (quicker is better obviously, but you have to give the plastic time to completely solidify which is where most of the time goes). When the mold opens, there is a series of ejector pins on one side that are hydraulically pushed through the surface to force the part out (this part changes for some molds. Really high end products, a robotic arm drops down into the mold and sucks the part out gently. Much slower and much more costly). These pins must be milled such that they're flush with the surface of the mold cavity.
Aluminum molds are common these days, but they wear out fast because they change dimension too much under temperature change. Steel molds are better and hardened tool steel molds are the best, but also the most expensive because you either have to mill them to slightly the wrong dimensions and they'll shrink during the tempering process or you have actually cut into tool steel which isn't fun (and usually involves high voltage Electron Beam Machining which actually uses controlled high-voltage current to molecularly explode the material you're trying to remove).
So you can see that the relatively craftlike process of casting tin soldiers in silicone molds is pretty cheap by comparison. Injection molding only pays off because of it's speed and the fact that the molding itself is almost zero labor. The tooling is so expensive though that you have to make thousands of whatever you're molding for it to be worth it.
