My snorkle mask has prescription lenses, and cost over $200. They sent it to me in a hard plastic case, so I keep it in there at all times, and put it in checked baggage. I have too much other stuff to lug around in my carry-on
My mask is not prescription, but it was $$. If it was prescription, I would pack it in my carry-on. But I put mine in my checked luggage, housed in a hard plastic case. I don't want the rubber to be crunched and distorted which might alter the fit. You can even pack a few small things in the plastic case with the mask....mask cleaner, extra straps, etc. to save space.