Don't trust too cheap glassess. The use of counterfeit glasses is worst than not using them at all. This is because when someone uses sunglasses their pupil dilates because the eyes feels like you are in a dark place. By means of that, the eyes become exposed to the sun radiation, increasing the nocive effect of UV rays.
Don't trust glasses that are too cheap. The usage of counterfeit glasses can cause more harm than good. When a person starts wearing sunglasses, the eyes "feel" like they are in a darker place, which induces to the pupil getting more dilated. The bigger the dilation gets, the higher becomes the exposition to the sun radiation, increasing the nocive effects of UV rays.