You can get cheap sunglasses at zennioptical.com. I bought four pairs of regular glasses for about $10 each. They work great. You can also get regular glasses and add on a cheap clip-on sunglasses piece for about $5. Prescription sunglasses are a bit more, probably in the $20 range.
Downsides, you need to get your prescription written down and submit that; you have to carefully measure your pupil distance, which isn't tough; it takes a while to get them (3-4 weeks). And, this last time I did have a problem with my order getting mixed up with someone else's, took about six weeks to get it sorted out. But I ended up with 8 pairs of glasses for $45.