I recommend against the use of 5.1 or surround sound headphones at any price range. Surround sound headphones generally rely on digital signal processing and lots of equalization which not only provides for relatively poor imaging in spite of the 5.1 designation, but also leads to a massive loss of signal quality.
I would suggest a pair of $100 closed headphones such as the HD280, DT250-80, or AKG K271S headphone. These models, as well as others, will provide for a significant leap in quality over any surround sound headphone.
Surround sound is sort of a joke amongst audiophiles, and only a few high end devices provide for relatively good surround sound emulation (such as AKG's hearo, but even that has serious problems associated with it). If you absolutely do want to go with a 5.1 system, at $75 to $150 your options are relatively limited. I believe that Zalman retails a pair of 5.1 headphones, but given that Zalman has never produced headphones before and probably doesn't know what they are doing, I'd go against buying these.
Really the bottom line is that no matter how good it's stacked up to be, 5.1 headphones are going to be a complete waste of money if you are after tonal balance, attention to detail, clear imaging, and a good soundstage. The idea behind "surround" isn't audiophile either, i.e., sticking the listener in the midst of the sound; this would be completely unrealistic if the recording was of a symphony orchestra and you wanted to hear the symphony in front of you. Even so-called "intimate" or "forward" headphones actually place the listener slightly in front of the orchestra in spite of how "forward" the sound is.
If it's any help, I've heard surround sound speaker systems and the sound like absolute crap. The sound is harsh, brittle, lacking in bass smoothness, tightness, or punch (a common problem with 5.1 systems is the darned .1, i.e., the subwoofer), lacking in midrange detail, and generally sounding about on par with $20 sony crap-buds.
I really hate to be so harsh when it comes to bashing 5.1 setups, but they are basically hype, just as most (but not all) equalizers, DSP technologies, and fancy plugins are pure hype.
Stick with the old school if you want good sound. If you want hyped and probably bad sound, then go for the 5.1 setup; while it's not guaranteed to sound bad, it's more likely to be less audiophile than the known good headphones on this forum in that price range are likely to be.
Sorry about the long rant.
Cheers,
Geek
