You really have to look inside the engine to come up with a close estimate. My mechanic friend had a 420 sel that the timing chain broke on and one cylinder wall was damaged when a piston shattered when the chain broke requiring a new silicone sleeve. This can push the price upward. Other than that, it was the normal parts and over 50 hours labor. While you are at it, change the valve stem seals at that mileage and have the heads checked for valve guide wear etc . When he was done with the repairs the bill to customer was about $3500 US