Cracked plastic of driver's side mirror

Today I was careless when backing up my II and hit the driver's side mirror on the corner of our rear deck.

The collision split the plastic of the mirror casing, which came out of place and turned the mirror forward.

I was able to turn the casing back to its correct position and the mirror can still be adjusted. It's not cold enough to know whether the heaters are still working. I couldn't push the lower part of the plastic casing back into place.

In the light of the morning I'll be able to get a better look at the damage.

So, can anybody tell me whether the plastic part of the mirror casing easily be replaced or would the whole thing have to be replaced? Could pushing hard on the casing to push it back in place cause any more damage? Anyone know how replacing the broken part should cost?

Any other advice? (I already know to be more careful in future ).

Update:
Morning came and I was able to get a better look. The bottom part of the plastic had dropped down. By pushing a car key underneath I was able to get it to go back into place: although it's not perfect at least it's not sticking out any more. The only obvious sign of a problem it the split across. If I turn the mirror away from its normal spot it'll pop out again. I guess I'll have to put some glue in the split or tape over it to keep the rain out. Fortunately it's a fine weekend here.

It looks like the plastic clicks into place, however, there appeared to be some damage to whatever it clicks into. So it probably needs two pieces replaced and will involve disassembling and reassembling the whole thing. I just hope the plastic can be ordered separately from the rest of the mirror.

I haven't yet had the car taken in to the dealer for the brake adjustment. Perhaps I can get an estimate then.