How much to fix left side mirror cover ONLY?

Hello, it's difficult to know what part of the mirror ultimately is needed, and more difficult yet: what is actually available for purchase? Sometimes, the manufacturer only sells the mirror assembly. And, even if smaller pieces are available separate, you'll need to determine if the housing is fractured or intact, and if parts that you can get are going to work. I would probably stop by a Mazda dealership for a visual evaluation with the parts department to see what can be ordered versus what is wrong with your mirror.