They can be prescribed also, one of my sunglasses was bought online and it cost me only $39, it is a bargain price for a pair of prescription sunglasses, the lenses are pretty good and the prescription is exact, I can see quite clear outdoors. For me, it is a waste of money to spend much on quality, instead, styles makes the importance.
Of course they can. There are some cheap prescription sunglasses. Prescription has nothing to do with the quality of the lens. What's more, some cheap sunglasses also have high quality. So if you need, you can ask your optician to make precription into your lens.