Cycling glasses are a very functional piece of equipment. While any glasses will be better than none, you ideally want glasses that:
- Wrap your face, so the frame does not obstruct your field of vision
- Have lenses that are close to your eyes, without large gaps between them and your face (to minimize dust and debris from getting to your eyes)
- Allow a reasonable amount of airflow to prevent fogging
- Fit well (meaning they essentially meet the above requirements without causing discomfort).
Cycling glasses will meet all of these aspects. However, the specific set you buy can range from low-end, nearly disposable glasses to high-end high-dollar ones. Features such as interchangeable lenses (so you can have clear lenses on shady days or in the dark) are nice-to-haves, and can be had even on cheap pairs. An alternative is to have a few pairs of glasses so you can pick the right tint for the day.
The right tint is very important--whether you have one pair with interchangable lenses, or several that you use depending on conditions, you'll want to ensure you have a pair for any riding time. Riding without glasses can be dangerous. Large bugs, branches, and road debris can really hurt if it catches you in the eye.
Price will partly be dictated by how long you'd like them to last. These will be taking bugs and other debris at relatively high speed, so they will all need to be replaced eventually.
Personally, I have a pair of relatively low-end Uvex Blaze's--paid about $30--and they come with clear and medium-tint lenses. They have suited me well, and have taken some drops. I'd expect I'll get perhaps a year of usage out of them before I do something disastrous and they are gone. The interchangable lenses are quite nice--I use them regularly, especially when commuting where I often need clear lenses after dusk.