I found my right side mirror on the ground shattered at about 4:30 this morning, called my apartment's management office, called police, etc. I'm sure they won't catch the person who did it, but that's sort of beside the point.

My question: How expensive is it to replace a side mirror on a 4th Gen? The actual mirror is the only part that seems to have significant damage on it.

 

If it is just the glass, look around and you can find replacments. Or a glass shop can cut a piece. Cost depends on what glass shop is doing it.
If it is the whole assembly check an aftermarket supplier.

 

if it's the whole assemblly, http://www.car-parts.com or http://www.partstrain.com/ShopByDepa...1KOV3EOTP2UCCS
but local autoparts stores such as Murray's or Autozone should have the glass.