Not all sunglasses, especially cheap sunglasses, block 100% of the UV spectrum. If sunglasses are just tinted but don't block UV light they're actually worse because the tint causes your pupil to dilate more than it otherwise would, allowing in more UV.

You won't find a pair of brand-name sunglasses like Oakleys or Ray Bans that don't block 100% of the UV spectrum.

That said, the lens quality between cheap and expensive sunglasses is significant. Clarity and contrast are improved with good lenses. They're noticeably worse with cheap ones.

Say the tint of your glasses reduces outdoor light levels to what would be comparable indoors. You'll see worse outside with your cheap sunglasses than you would indoors without. Cheap lenses distort. A good lens doesn't, and make things look sharper and better defined by increasing contrast, optimizing for desirable light wavelengths (e.g. yellow lenses when skiing in flat light), polarization to cut glare, etc.

Added bonus, sports-oriented glasses like Oakleys are shatter-proof. This is a significant consideration for me, as I participate in many outdoor sports where eye protection is mandatory.

Bottom line, price does make a significant difference in sunglasses.