afaik.. bmw use alpine clones for their changers.. a dealer price of 350 is typical.
i very much doubt youll be able to buy a changer straight out and plug it into the bmw lead (i could be wrong). so you could try your local ice dealer (or halfords) and see if they do a changer package with the adapter lead, this would probably be around the £230 mark.
im not totally sure on the 5 series but the 8 series have the changers all built into a panel in the boot which restricts the size of the changer, just remember that when buying a new "non bmw" changer