Drewcifer - you might be right about the 'common materials' part, but you could be wrong too. I don't have the money to buy and tear down a 500$ helmet but not all poly is the same, and out shells are certainly not all the same.
You can get the polystyrene in various densities, and I am sure you can splice it with other chemicals that make it more or less 'rubbery' so that it gives more rebound after the initial compaction.
Shells on the other hand can be anything from cheep plastic to, reinforced, ribbed, fiberglass, carbon fibre or whatever else the engineers dream up. To me that would matter because you aren't going to just hit your head once in a crash and its over, your gonna have a series of bumbs and scraps and hits from odd angles, and even if the poly compacts i would like that outer layer to stay together on my head.
After all of that, you pay for better venting design, how light it is, visor quality, and inner padding quality, plus durability to weathering.
My helmet was only 150$ but its an icon so the engineering ideas are passed down from their more costly helmets, but if i was to go with a no-name from amazon, i can only imagine how uncomfortable it would be to wear, and how foggy the visor would get or how hot without good airflow..
So many things can be messed up, and TBH i think thats where most of the money goes in helmets. I also think at some point you reach the point where the benefits outweigh the cost increase and you need to realize that a 750$ may not be worth the extra money over the 500$.. or the 400$ may not be worth it over the 250$ model.