Not many good pair of headphones for $20. I hear that Sennheiser's HD 485s and JVC Flats are pretty good for the price range, but I don't think they can really compare with the Beats by Dre set (I don't think you need to spend $300 for a good set either. I'm really happy with my Sennheiser PX 210 BT that I got for less than half that)
I mostly agree with this, but it depends on what you're willing to accept as a "good" pair of headphones. There are a lot of people who would be satisfied with $20 headphones.
Personally, I think the vast majority of people out there would be satisfied with the ATH-M30s or the Samson SR850s which are still pretty cheap (unless you're a bass head), they just don't know they exist and instead go to Bose/Beats...