How much can it cost to do Oil Pump replacement on a 300D 2.5 Turbo??

I ended up taking my car Mercedes Benz Canada because no mechanic [I tried 3 different places] could not fix the lifter noise issue that I'm having, all kinds of oil treatment, using the right diesel spec oil etc, replacing all 10 lifters didnt' fix the problem, the oil pressure guage readings are ok. MB thinks its the oil pump beacuse oil is not getting to the lifters, I asked why is the guage showing good readings, MB answered the guage is at the bottom of the engine close to where the oil pump is so pressure might be fine at the bottom but surely not getting to the top. They haven't called back with the number on how much its going to cost so I just wanted to get an idea to compare, I understand MB will be more than some other shop, unfortunately I haven't had any luck with the best of the best non MB mechanic in the greater toronto area so am willing to pay the additional price.

Thanks,
Atif