Definitely shop around - even between MB dealers. Give the mileage to your dealer and ask that they tell you which additional items will be required, and ask for a breakdown for the service itself, and then each additional item separately.
Additional items are the mileage related stuff like spark plugs (not on a diesel though ), brake fluid change, transmission fluid change, etc.
I did it for my C32 in the Summer of 2009 and found this variety - all are MB main dealers:
Most expensive (an MBUK owned MB main dealer): £980
B-Service: £495
Fuel & air filters: £310
Brake fluid: £175
My usual (franchised MB main dealer): £459
B-Service: £280
Fuel & air filters: £127
Brake fluid: £52
Other (franchised MB main dealer from a different group): £445
B-Service: £275
Fuel & air filters: £125
Brake fluid: £45
I ended up using my usual franchised MB main dealer. Actual final price was £379 for B-Service, all filters including fuel filters, brake fluid change, ATF level check, STAR scan, valet (a good job too), and a thorough shakedown.
I supplied the oil which cost £43 for 8 litres, which is about half the cost of buying the oil from the main dealer. If I'd not needed the additional items - only the B-Service - and supplied my own oil then it would have cost £199, plus the £43 for oil from CostCo.
The two dealers closest to home - one MB Retail one MB Franchise - both refused to budge on price, weren't helpful when I called and quoted more than double than I actually paid.
Should make a mental note to call dealers tomorrow...