This from my dealer:

-12 month extension - $2,500
-24 month extension - $3,200
-36 month extension - $4,120

-This warranty is backed by Mercedes-Benz USA, and has essentially the same coverages as the basic warranty.
-The 12/24/36 month extensions all have a mileage limitation of 100,000 total miles on the vehicle. So, if you purchase just a 12 month extension - you only have one year's warranty/or until you reach 100,000 total miles on the odometer - WHICHEVER COMES FIRST!
-You are able to extend the warranty anytime the vehicle still has the factory warranty on it (4 years or 50,000 miles). However, if you wait until the 2nd, 3rd or 4th year, MBUSA charges an additional 10, 15, or 20% additional premium over and above the normal cost of the extended warranty. Mercedes-Benz of Portland encourages you to purchase the warranty within the month you purchase your vehicle - and gives you added incentive to do so. If you purchase your car in the month of June (for example), and decide to purchase the extended warranty by June 30th, we will back it will a 100% Full Refund. Details on the full refund offer can be given to you for further review - however if you do not have to utilize the extended warranty you purchased - at the conclusion of the extended warranty period, we would refund you the amount you paid for the extended warranty.

 

I got a quote for $3570 for 3 years 100k miles
extended warranty. Call their Finance manager, have VIN ready
for more accurate quote.