Are cheap sunglasses just as good as designer sunglasses?
Sunglasses should have a CE mark which shows that they meet the European Standard for sunglass protection. Sunglasses which don't carry this mark or a UV400 label will not protect your eyes from UV radiation and so aren't worth buying.
When you wear tinted lenses your pupils open wider (your pupil decreases in size in bright light) so if your sunglasses don't provide the necessary UV protection your eyes will be exposed to more UV than usual, increasing damage.
BS EN ISO 12312-1:2013+A1:2015, Eye and face protection -Sunglasses and related eyewear and EN ISO 12311: 2013 – Personal Protective Equipment – Test Methods for Sunglasses and Related Equipment.