The part is around $75 to $90. You can replace it easily. You should be able to remove the nut on the strut shaft. Then press the shaft down until it clears the bottom of the mount and moves to the side. Remove and replace the mount. Then using some creative tooling like a large straight slot screwdriver and maybe some vice-grips with a red shop rag around the shaft, maneuver the shaft back into the mount and tighten the nut.

$500 is ridiculous, unless there is something about the 4-matic front suspension that is radically different from a normal 124.

Steve