This is just my opinion, I think that cheap glasses is worth buying than the expensive one. Based on my experience, I've been using glasses for nearly my whole life. From eye health perspective, my minus eyes don't worsen even when using cheap glasses, well maybe just had a plus because of my age.
So I've been and still using an expensive glasses and in my opinion it's the same as the cheap one well maybe better quality and that's just a bit better. I could buy 4 cheap glasses instead of buying one expensive glasses, my benchmark is endurance and quality... as for the frame it's still has a flawed like loose screw the paint coming off, it's just not worth it.
To see the difference, I'll show the spec that i use for the expensive one with the same price budget
- Expensive - Semi high quality glass, no auto shade, good strong frame.
- Cheap - High quality glass, auto shade, good strong frame. ( I could afford to buy 4 glasses )
As you can see on top, maybe the difference are only in the quality of the frame and auto shade, as you can see with cheap glass i could get a higher quality glass with an auto shade ( meaning when the sun showing it will dim the glass ). If maybe I bought four cheap glasses, I could change everyday with the glasses I feel like, maybe I feel like using sporty glasses because I wanna play basketball, maybe I wanna change when going to a meeting etc...
So are cheap glasses safe? I've been using for years and its safe
How are cheap quality glasses? In my opinion, it's just the same but maybe its the difference are in the frame and where you buy it. Well maybe the place I bought the glass has better checkup system, and that is the only value that it's worth using everything else for the glasses are the same.