Huh?? Please 'splain.

The smart key remote intelligence (lock/unlock) infrared and rf is battery powered. The engine start and unlock is powered by inductive energy (from the key collar) in the car. The circuit board slides out of the housing after releasing the small catch in the metal key slot. It slides out similar to a match box.
As everyone knows the smart key can not be reprogrammed but if you have a slightly damaged housing you can swap your circuit board into one of those E-Bay remotes and throw the rest away.MB has warned about having too much weight, keys and junk putting a strain on the catch. Of course you dont know what you get with an E-Bay remote. The new one ,chrome, is much nicer and under $200

This was the price when I last ordered one about a year ago:

The remote (20376670069999) is $105.00
The blade (20376601069999) is $17.25

I got mine from Steve Foster at Duval in Florida and his price was impossible to beat. At the time my local dealer wanted $170 for the remote alone.

If you don't order a new blade and only have the old style key you will not be able to put the old blade into it so you do need both parts. Also, the first time they sent the blade it would lock the car but not unlock it so it had to be replaced. Moral to the story: check those "emergency" keys - don't assume they work.

Oh boy MB got me also, when my old remote for my 98 E320 broke the dealer got me for $160. The new one looks great with all the chrome but it is bigger than my old remotes, that thing is huge in my pocket and wish it was a bit smaller. I think the price was not too bad as I was expecting to pay at least $300 a new unit, in the past they were much more costly...They did however cut the mechanical key and included it with the remote but that key refuses to open the door. If the dealer tells you they can't fit the mechanical key they are full of it as my dealer cut and fitted the key in my new chrome smart key.