I know prices may be higher but check this: https://www.techwalls.com/production-costs-of-smartphones

If you go to china and go to a factory they will show you the menu where you can choose the components from, you just give them the 3d cad file of the desired form factor. And prices are the one I linked. You have to customize android, I know, and spend on marketing, but that's it

"Big companies sell their high end phones for cheap"

What crack rock are you sleeping under? I can buy a street legal used vehicle for less than the "latest and greatest" samsung or Apple phone.