How much is a replacement key for a 1999 Mercedes e320?

Take the spare/valet key out of it's holder on the fob. In the slot, under where the head of the valet key is stored, is a grey slide lock. Using the valet key or a similar tool, slide the gray lock under the body of the rest of the fob. This will unlock and push up the top of the fob. Pull out the internals and replace the battery. Two CR2025, in my key.