I broke mine and got a glass guy to put a piece of regular flat mirror in.
There are two things to consider.
1. The OEM mirror is, liker all vehicular mirrors, only partially reflective- that stops you copping the full glare of headlights from behind at night.
2. The OEM is slightly curved to give a wider fields of view.
There is one advantage in the flat mirror- it gives you a true indication of how far behind you the guy in the car/ truck actually is.
And, I guess, there's a fair bit of money difference. The glass guy charged me $25. But then he's a rider, and therefore a mate.