$100 is tight...if you can find a good pair use, by all means...
That said, Grado tend to be cheaper but require more from the amp.
I have owned Senn HD580's and HD600's, and I love them. They are more than twice your budget new, who knows used.
Beyemeiser phones are spse to be the bomb, and look into AKG.
Just pay attention to the impedance... Senns are 600ohms, Grados are 32ohms...the lower the harder for an amp to drive...i.e., the more you'll have to pay for an amp to due justice to them.