Generally the control arms go bad. Have a friend drive up and stand on the brakes, if the front tires shift forward and you get a thunk - the arms are bad.
OEM run for $129 each. Alternatively you can replace the bushing, but by the time you pay someone to press out the old and press in the new it's easier to just buy the arms new. Not to mention the fact that the ball joint on the other end will be worn as well, and you can't replace that. That's why I just replace the whole arm.
BMW X5 Front Control Arm / Tension Strut OEM