on my motorcycle, a 2013 Yamaha YZF-R6, and I suspect many other sports bikes, the rear view mirrors are more or less useless. Being rear-ended by a vehicle is also one of the most common reasons for motorcycle related fatalities.
Skully, a helmet manufacturer has been developing a helmet with built in display but retail price is $1500 and to the best of my knowledge they still have yet to begin shipping to customers. I had originally preordered one until I finally got tired of all their excuses and delays and cancelled my order.
I don't care about being about to see data from my phone like the Skully system is supposedly capable of. All I wanted was to be able to have eyes in the back of my head so that's why I built this very simple system from pre existing components with minimal knowledge in electronics. Grand total was under $100 in parts.