Although you imply that the mirror was merely dislodged, it is most likely that its plastic base was snapped. If that's the case the whole unit must be replaced.
The cost of a new unit varies considerably. You haven't told us if this mirror is the type that is operated manually or electrically. Also, a replacement puchased from a dealership may well cost twice as much as a mail-order aftermarket unit.
As a guess, expect a total repair bill of $150-$200, and it would not surprise me to hear a dealership wants even more. If you were able to shop around you could do better by finding a guy who does driveway repairs but I guess this is not an option.