The Koss KSC 75 is considered one of the best headphones in the cheap range for portability. (
http://www.amazon.com/Koss-KSC75-Portable-Stereophone-Headphones/dp/B0006B486K)
My eyes were opened with a Sennheiser HD201 I got on sale for $12, but the Koss was pretty much the closest thing I could get to a PortaPro (the best Koss headphones) without spending $50. Also, a full sized headphone is probably not going to get enough power from a 3DS without an amp. It's $20 but regularly goes on sale. I've gotten them at Kmart for $10
Everything else is pretty much the same.
http://gizmodo.com/5617200/the-secret-scam-of-cheap-earbuds
If you're really looking into headphones, check out head-fi.org for their reccomendations.
Also: Beats/Soul/Rapper headphones are double if not triple the price of a comparable headphone from another manufacturer.