Wow, I have almost the same car (C Class coupe) and also have one key and have been fearing about the possibility of losing my key since I bought it. The seller only had one key and from research I did 2 months ago, I found that it'll cost around £200-£350 to get a spare key for a car.
It would cost even more if I lost my key as Mercedes will need to reprogram the car's lock system to a new one and issue you with new keys and unlock the vehicle + they have to go through a few verification procedures to confirm that you are actually the owner of the car and go through a few admin procedures.
As far as I know, Mercedes are the only people who can provide you with a spare key that works. Many (if not all) third party key suppliers/programmers, especially those on eBay, who claim that they can program a spare key are fakes. They may unlock/lock the vehicle, but when you put the key in the ignition and turn the key, the vehicle will not start as the immobiliser will activate. They'll need the computer to actually program the vehicle's central locking system and immobiliser to work with the key.
Similarly, blank keys on eBay will not work as a spare key (though it will work if you take the chip inside the key and put it into the new housing).
One thing I think you could do is to just get the emergency key part of the key cut in a key cutting shop - but that will only allow you to access the vehicle in case you're locked out but not start the vehicle. I'm yet to do this.
I'd recommend checking out various Mercedes forums where someone can better help out and share their experience/knowledge in the matter. MB World(mainly US), Mercedes Club (mainly UK) and Benz World are some popular forums to search.