Headphones under 40$

My favorite headphone under $40 is the JVC HAS160. It is around $11 on Amazon. It is closed supraaural, however it doesn't isolate so well. If you are willing to spend $41, then look at the Sennheiser PX100-ii. You can get the Koss KTXpro1 for around $15. If you don't like how it looks, you can get the Koss KSC75, which has similar drivers, but no headband, so the bass will be less. Another choice is JVC HAS600 for around $30 on Amazon. . It is on the ear for some people, while for those with smaller ears it is over the ear. It is an extra bass headphone, so the midrange is somewhat recessed as are the highs.It is a closed headphone.

You didn't mention it you want a closed or open headphone, and if you want a neutral one or an extra bass one.

The koss portapro should fit your bill given its portability and sound quality that suits most genres of music. 

I'm listening to a set of JVC has160's as I type and they are really nice for $10 and are very portable and easily driven so if you want to really cheap out, they are a very good set. Other ones to consider are the Panasonic RP-HTX7, which if I recall right, are a bit bass heavy, but also larger. Another option if you have patience, sign up for Newegg's e-mail list as every couple of months they discount the Meelectronics HT-21 to under $20 which is a good set at that price level.

I'm partial to the JVC HAS600 in that price range.

The HAS600 is good, especially after extensive burn in, however the HAS650 is much better. The HAS650 has more detail and tighter bass.

Im kind of on the same boat looking for an ultra portable. Porta Pros or Px100 - iis? On amazon they are almost the same price. I know people say you could get the porta pros for $30, if I could find a pair for that price then id def get them.

Imo you should skip the Portapro. I have had the Portapro for over 10 years. I also have the PX100(the original) which is much better. Even better than the PX100 though is the JVC HAS650($34) The HAS650 doesn't fold like the PX100II, however it driver enclosures rotate 90 degrees, making the headphone flat. The HAS650 is closed, but very comfortable and compact. Imo either the PX100Ii or the HAS650 would be a good choice. The PX100II folds small to fit in a jacket pocket, while the HAS650 has slightly better sound. Both have an overall warm sound signature. While the Portapro also has a warm sound, it bass is not well controlled.