Despite warnings from my colleagues, I do much--but not all--of my mixing using phones because I'm dealing with a noise issue in my home studio. Folks are right when they tell you to avoid this: the sound is different in cans and speakers. That said, though, I'd add that you can do much of the rough mix in phones, especially if you become *very familiar* with your phones by listening to lots of well recorded music through them (e.g., Steely Dan's latest). Then I connect my DAW to my home stereo and make all of my final adjustments, especially related to EQ. So yes, do your final mix using some type of external monitor, but (in my opinion) you can use a good set of phones to get your sound in the ballpark--if you have little choice. My final recordings, using
a Yamaha aw16G, sound surprisingly professional if you AB them
with commercial CDs, and that's the approach I use.
So which phones? Most people would agree that there are several in the $100 range that are quite good, depending on your personal tastes: AKG, Sennheiser, AT, and Sony all make popular, highly rated models for about $99. I'd suggest trying them in a store while using a CD that you know well. Better yet, although a bit impolite to some, you could order several models from an online store and then return all but your favorites. I have to admit that I do this occasionally because I feel the need. (Shame on me?)
Good luck with your decision!
J.