For $100 I would probably go for a pair of ATH-M50's. But probably best to go for something a little better and preferably open if you want a more natural sound.
I finally decided to buy a pair of Grado SR80i open-back phones, and I gotta say, they sound pretty damn good, better than any headphones I've ever had. I got them for $99 from Parts Express. Thanks to everyone for your input!
Bough ATM-50H on a recommendation, pretty much useless for anything other than airplane travel IMO.
Recently bought Sennheiser HD650 ($305 delivered to my home same day from Amazon!). I love them.