My 99 S420 just came from the shop for fixing its MAF. Eventhough, I still have the 4 year/50k mile warranty left; they said it was covered under the emmisions warranty of 8 years. My MAF sensor was defective causing both $2500 a peice cats to go bad and fouling the 02 sensors. Since my car originally was from Canada, the rust had glued the exhaust causing them to replace those components (both resonator and muffler), too. Make them try to fix it under the warranty but I know they probably know a way to get yout to pay $82 an hour. What was strange in my case is that I never got a check engine light from the defective parts. I just smelled sulfur while driving normally.
I think my job taught Mercedes a lesson to try to make better MAF sensors. I think the heat kills these sensors. In the m119 engine, the sensor sits in the V of the engine and in my M104 engine it is right above the headers. My 97 E320 has only 38k miles, so I am waiting to see the Check Engine Electronic sign.
If they squiggle themselves out of covering it, then pay them anyways because you don't want to be forced to get new cats or O2 sensors, down the road due to a inexperinenced mechanic. I think maximum amount you might be paying is $600. Ask them if you can buy the MAF sensor from fastlane and tell them to install it.