The knuckle is part of the front suspension. It has a spindle for the hub to ride on. It attaches to the suspension at the back with the strut and the bottom with the ball joint. A steering arm and tie rod connect to it for steering and the brake carrier attaches to it for supporting the brake caliper. For most W124's, I believe the part is aobut $220. Either the 500E is WAY more expensive, they are overcharging an extreme amount, or that's labor to install or both!