A wind deflector is essential equipment in a convertible, so it should come standard in my opinion. I had to buy one, because BMW.
I acquired a second-hand wind deflector, bought from a lovely lass in Germany. These are essential equipment in a convertible, so it should come standard in my opinion. It folds up and fits in the boot, which is nice, and only set me back $350. A new one is around $1,600.
All of the aftermarket ones I looked at, which would have been around the same price I paid, didn't fold to fit exactly the boot size, so would need to be stored in the shed when not in use, not with the car. This OEM second hand one, in new condition was a good find.