At that price point there are several options. If you want to spend a little more money, you can get Grado SR60s (which i believe are $70). These are the headphones that I have and I love them - great clarity, nice bass, fairly good range.
For $50, you can get the Sennheiser PX100 (does anyone know anything about the PX200? I read a little thing about it and they basically said the 200 wasn't as good as the 100...). The PX100 is regarded as probably the best $50-or-under headphone, and rivals the SR60 in sound quality. A little older than the PX100 is the Koss PortaPros, weighing in at $50 (I believe). It's debatable whether the PX100 or the PortaPro is better, but they're about the same in terms of quality.
On the lower end, Koss rules the roost - if you can find a discontinued KSC35 for around $30, these rival the sound quality of the $50 headphones I just mentioned and is cheaper to boot. KSC50s are also basically KSC35s in a different enclosure - I know very little about the KSC50s, however.
On the VERY low end, Koss has another pair of headphones - the KSC75. The little brother to the KSC35, these are regarded as the best headphones in terms of value in the audiophile world - many audiophiles swear by these in a portable system, but they'll suffice in a computer setup as well. You can currently get them for $20 on Amazon, shipped. I have a pair of these arriving soon and I can't wait!
Enjoy.