FWIW, I got a quote for a new key from my local MB dealer here in SW PA. My car is a '97 E300D NA. I currently have only 1 completely functional key/fob. They told me it would cost around $300. Which, considering all it does and what would be the alternative to getting "stuck" with my one and only key nonfunctional (lost, broken, etc.), I didn't think was too bad a price. I do have a second fob. It's broken, won't unlock/lock doors with the "button". However, the "metal" key, in the broken plastic non-functioning fob, will work in the ignition and trunk (unlocking the car).
For some things, I think sometimes, ya just gotta bite the bullet.....