Rx glasses are almost always cheaper compared to US mainstream prices. However you often get an inferior product without knowing in HK - sub standard lenses, extra thin lenses that don't meet safety standards in other countries, inappropriate lenses for the frame(e.g.rimless) - I guess you'll know eventually when you visit your optometrist or your extra thin lens cracks for no good reason.

HK prices are comparable to US prices at Chinatown shops. Also consumer protection and optician licensing is usually of a much higher standard in the US.

Shopping online is often not practical.

I stopped buying Rx glasses in HK many years ago, going to Chinese malls in Canada instead.