The TEMA kit spacers are made from some type of hard rubber. The front spacers have a metal insert for the bolts but the rear spacers are all hard rubber.

This might add a bit of compliance to the suspension which could be good for ride comfort on rocky roads but not optimal for handling. I have other cars which are optimized for handling (like a classic air cooled Porsche 911) so this is a tradeoff that I am willing to make to gain some off road capability in the X5.

And I'm pretty sure that is where most of the cost difference comes from (molded rubber vs machined aluminum). The other likely contributor to price difference is country of origin and their relative native costs (Tema is Russian made vs FMU is US made).