Two choices: #1 Call local MB dealer and see how much they get over the counter for new battery for your car. Could be cheaper than you think, but surely over $100.
#2. Call local Interstate Battery distributor (not dealer) and see what they get over the counter for battery for your car. Dealers sell the same batteries, but charge more since they are in the installation business. I hate to put a non-MB part in my car, but when daughters 300D needed one, Interstate it was. Her car is not exactly a "show car" anyway so I didn't mind having a non-MB battery visible under the hood.
You really should do some testing though to see if something is draining your battery while your car is at rest. Try putting an ammeter in series between the negative cable and battery terminal and see if you have any current draw with car off and doors closed. Anything more than say 50 mA requires looking into. Just pull fuses one at a time till the current draw go away. That should help track down the drain.