Okay, do you have some numbers to support the start-up cost assertion (or the volume assertion, for that matter, though I'm inclined to agree with you on that)?
I used to be involved in Mage Knight (and some of WizKids other games) and ended up in a discussion online with someone who used to work in making plastic miniatures (or something very similar). This is the information I gathered from that, and it hasn't been contradicted in the many times I've discussed it since then.
The chief difference between plastic and metal miniatures is in where the cost of production lies. A metal miniature's material is a chief cost. The mold is pretty cheap (being a rubber or similar material mold), but the metal is more expensive. With a plastic miniature the material is cheaper. However, the mold is
much more expense. The number he gave was in British Pounds, but worked out in the multiple of thousands of dollars (and he stated he worked with plastics during the 80s) for a mold.
Given that, to justify plastic miniatures you need a very large print run. You need to spread that cost out over many miniatures. Metal miniatures don't have that huge cost to initially create a miniature.
Now costs likely came down since then for a plastic miniature mold (if it went up it would likely be in the tens of thousands per mold given the cost he quoted). However, remember that there are often many molds to make the parts that go into a miniature. If you look at WizKids miniatures you will see that they often reused certain parts over and over (particularly in their Mechwarrior line). That fits with it being more expense to create a new mold for a miniature (or piece of a miniature). The more you can reuse the mold, the less the expense.
That also ties into Merric's law*. Part of the reason for random distribution of miniatures is to make sure it is economical to create the larger number of miniatures (you have less worry about making a miniature in numbers that sit on shelves while others sell). When you remove that ability you either need to increase cost (to balance out those miniatures you created that don't sell in large enough numbers) or have less choice (because the manufacturers can only create miniatures that are certain to sell in large numbers).
* Non-Random Packaging, Cheap Prices, and a Large Range of Figures: Choose two.