I probably misunderstood which subwoofer you are talking about, but the OE subwoofer that goes into the right pocket in the trunk area is about $240 with free shipping from the BMW of Fairfax. from my understanding, it inclludes the speakers...

part # 65138379378.

you will also need some hardware to attach it, 65106905660, 07119905720, and two of 16131183133.

that thing is already molded to fit in the space, and there should be mounting points on the body of the car as well...