They are not that much more expensive. Less than £40 each as I recall from the last time I replaced some that had fallen to bits.
The actual drivers are not very expensive in most speaker systems although the quality that B&O uses is particularly unimpressive. I would say the exception to this statement is the so called woofers in the CX range which always seem over specified to me. Apart from the foam rot of course.
The thing about B&O is the bits are cheap and it's the packaging that's expensive!