Electronic car key

- Replace the battery
- Take it to the car dealer

If you need a new key + brain to go with it, you could be looking at several thousand euros. A battery will be a few cents.

I guess it depends on exactly how it is broken but if you mean physically damaged as opposed to just needing a new battery, I paid around €80.00 to get mine sorted out. It will have to be done at a main dealer and, for some obscure reason, they will need the spare as well. And it's not a matter of ordering a new one and then going to collect it, it actually involves someone working on the car for about 20 minutes so.

250 euro for a new key for Mercedes