I guess if a single cavity mold costs $30,000 and a 16 cavity mold costs $110,000 you have an additional expense of ~$80,000, divided over half million parts is 16 cents. So lets say somewhere from 5-10cents per part to go 16x faster. My numbers might be off a bit but seems in the ballpark. I also don't know much a lego brick costs to make in terms of materials/opex.

In this context, a cavity produces one part. A cavity can only produce, lets say, 200k parts. If you produce 400k parts, you only need a single mold, but you've only used 25k wear on each cavity, but you paid more for the mold instead of only 2 of the cheaper single-cavity molds.