the material does cost alot but thats not what makes it so expensive
the sheer amount of machine and welding work required is what makes them so exspensive.
i will say sheetmetal manifolds are over priced
example
it used to be to make a intake runner u would machine out the shape out of a steel block and form ur aluminum around it to make each runner.this alone would take hours per runner.and that was just to make a runner that wasnt even welded to the inatke or plenum yet
today they just take a chunck of aluminum and throw it in a cnc machine and machine it out into a billet runner pretty much