The best deal I've found- 3 Pairs Kickback Foam Padded Motorcycle Sunglasses https://www.amazon.com/dp/B003FBV8ZW/ref=cm_sw_r_awd_KAwcvb044MVF9
3 pairs for under $20 and they work better for me than any others I've tried. Same company also makes ones with transition lenses for $30 on amazon. Those are my everyday riding glasses.
$22 for 3 pair. There is a transition version I have that's great too, $30 something for those.
I just flip my visor up and put on a pair of clear padded glasses when its get dark. Seems a bit easier than carrying another visor.
i bought these a few years back and i dig them.
http://www.amazon.com/Pairs-Kickback-Padded-Motorcycle-Sunglasses/dp/B003FBV8ZW
i ordered this set a while back
http://www.amazon.com/Pairs-Kickback-Padded-Motorcycle-Sunglasses/dp/B003FBV8ZW/