BMW x5 2002 XENON light

Hi
I have an x5 2002 3.0d with xenon headlights. One of them went off recently, but prior to it going off it went from bright light blue to more kind of pink color. BMW dealers are quoting me over £180 for the bulb or over 400 for a ''control unit'' depending on which one it is. Is there an easier cheaper way to replace this or found out which one is more likely to be at fault? Thanks