Most of the original (boxed) CPU fans are machined. The sheet metal fans are 3rd party and they need costly heatpipes to move the heat away from the (machined) CPU base. It will be costly because the machined metal fan will need great balance and a good polish to stay quietly floating above the base and not jump and rip through the computer's case.

You cannot use plastic as it has a lousy thermal conductivity. You cannot use copper because great thermal conductivity (2x better than Al) comes with greater weight (3 times the density of Al).