You don't need to spend a lot of money, but you do need 2 things, and I strongly urge you to get the 3rd.

100% UVA and 100% UVB are MANDITORY. You CAN find this in $15 sunglasses at walmart, but not all sunglasses in any price range have this. Make sure the glasses are labeled as 100% for both. If it's not labeled that way, don't buy it. I don't know if it's still true, but years ago, there were 3 categories of sunglasses, "cosmetic", "outdoor", and something else that I don't recall. Those ratings were based on the UV protection. Cosmetic only had like 50-60%, not good. The ones rated for extended outdoor use were 100%.

The last thing that's optional, but I find really helps is polarization. This greatly reduces glare, maks it possible to fly nearer to the sun (or not loose site of your plane quite as far away from the sun), improves depth perception, and frankly makes it easier to see. I find myself squinting with out polarized lenses, but since I've started wearing polarized glasses, I'm much more comfortable outside for a long period of time. (fwiw, I think I'm paying about $50 for a pair of glasses. Not as cheap as possible, but not really expensive either. And they came with a hard case that keeps me from breaking them when I don't have them on)

A couple of other features that are worth looking at:

- color of lenses. I have two pair of glasses, one is a dark gray for bright days, and the other is a lighter brown for clouds. (I'd like it to be lighter, almost yellow, but brown was what was available when I was ordering). Lighter tint glasses give you the UV and polarization benifits on cloudy days while still giving you good color and depth perception. The UV and glare is still present though clouds, so it's worth having around.

- optical purity. If you take a pair of cheap glasses and hold them away from your face and look at something like a tile floor (lots of long straight lines), you will sometimes see the lines become non-straight at the edges of the lenses. Quality glasses should have absolutly no distortion anywhere on the lens. For flying this may or may not matter to you.

- field of view. The larger the lens and the location of the lens affects how much you can see to the sides and how easy it is to get the sun-from-the-side problems

- safety glass. Having something that protects your eyes from bits of broken prop or fuel accidently squirted is really nice. I've lost count of hte number of times I've gotten squirted in the face by fuel. It sucks a lot less when wearing glasses, but some don't protect as well as others, I wouldn't count on cheap glasses to stop much of anything other than liquids.