Well almost nothing in this world is priced based solely on the cost of materials so I wouldn’t waste your time thinking in terms like that. And it is in fact the case that a $500 router is demonstrably better than a $100 router. A $300 UniFi router is pretty much the ground floor of decent router performance, and even then you’re severely lacking in warranty and support, and the software is subpar.

Beyond a certain point things that are more expensive are just more expensive because they are, not because they represent better quality.

This is true, but the “certain point” in this case is not $100. $500 is much closer to being that point, and even then that’s only if you’re thinking in the scope of a consumer router. Business class routers are thousands of dollars.