The DIY approach can yield really amazing results much higher quality than commercial phones in this price range. I discovered the Fostex T50Rp ($80) was a modder's paradise, and cut my teeth on it on head-fi here (I have a business doing Fostex mods, but it is unrelated to this thread):
Another option is to forgo headphones and use in-ear monitors (IEMs). IEMs under $100 can sound way better than comparably priced headphones. Check out products from Shure, Velodyne, etc.