Shop at some independents and check some parts stores for prices. The aftermarket sells rebuilt half-shafts for most cars, but the 4-Matic was not a high volume model, so maybe rebuilt half-shafts aren't available.

I lost a CV-joint boot on a VW Rabbit over 20 years ago, but I caught it right away and took it to the dealer. They removed the shaft, and there was no damage to the CV joint, so they repacked the joint and assembled it with a new boot. It wasn't too expensive, and I never had another problem with it while I owned the car.

Duke