The most expensive part of the upgrade is the rotors because you have to get the BMW ones (395mm). There is a less expensive route using the Range Rover 6 piston calipers but requires slight modifications. It uses a 380mm rotor but you have way more vendor choices and considerably less than the BMW rotors. I've got a set of the 6 piston Range Rover Brembos and new 380mm rotors that I will be offering up for sale for someon that wants to pursu this route.