What do you call the lenses which can change to sunglasses when you go outside? I wanna buy this kind lenses from the online optical stores. Do you know the average cost for them? Thanks for your kind help.

We call them photochromic lenses, they are clear indoors and darken automatically in response to sunlight outdoors. I have no idea about the average cost for them at the online stores, I just bought a pair of photochromic prescription glasses at the retail store and it cost me more than USD200.00.