If you're going the 'cheap' route, you might consider tinted safety glasses (Home Depot, Walmart, Lowes, pretty much anywhere has them for around $5-10) they tend to be tough, flexible, fit well under helmets and if you lose them, its no biggie. This is what I use for most of my riding (I also use clear for winter/night/etc...) -- I bought 10 pairs (mix of clear and tinted) from costco.com for around $32 including shipping...

That said - I prefer good sunglasses - they really do make a difference, and your eyes will know the difference. But they require more care and respect that I can always give them for commuting. Thus the cheapos.