You can buy just the mirror glass. Make sure you order the right glass, since you can have the standard non auto-dimming and one that has auto-dimming. I am not sure about the price, but you can check with Pacific BMW and/or Crevier BMW. They seem to be two of the best places to buy OEM parts.

I am going to PM you the instructions to remove the mirror glass. Didn't want to post it here, if you know what I mean.